Troubleshooting trace inspired by:
http://live.gnome.org/NetworkManager/Debugging

A Good Bug Report... ref:
https://bugzilla.gnome.org/show_bug.cgi?id=665630

your distribution name and version:	Kubuntu 11.10

your kernel version:			3.0.0-15-generic

your network device's brand and model:	Huawey E156G

your network device's hardware IDs
if it's a USB device, include the output of lsusb -v:

*********************
Bus 001 Device 002: ID 12d1:1003 Huawei Technologies Co., Ltd. E220 HSDPA Modem / E230/E270 HSDPA/HSUPA Modem
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  idVendor           0x12d1 Huawei Technologies Co., Ltd.
  idProduct          0x1003 E220 HSDPA Modem / E230/E270 HSDPA/HSUPA Modem
  bcdDevice            0.00
  iManufacturer           2 
  iProduct                1 
  iSerial                 0 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength          108
    bNumInterfaces          4
    bConfigurationValue     1
    iConfiguration          3 
    bmAttributes         0xe0
      Self Powered
      Remote Wakeup
    MaxPower              500mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           3
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               5
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x82  EP 2 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x01  EP 1 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        1
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x83  EP 3 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x02  EP 2 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        2
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass         8 Mass Storage
      bInterfaceSubClass      6 SCSI
      bInterfaceProtocol     80 Bulk (Zip)
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x84  EP 4 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x03  EP 3 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        3
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass         8 Mass Storage
      bInterfaceSubClass      6 SCSI
      bInterfaceProtocol     80 Bulk (Zip)
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x04  EP 4 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x85  EP 5 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
**************************

the output of the 'dmesg' command from around the time you experienced your problem:
$ dmesg | tail
[ 1942.312397] usb 1-3: generic converter now attached to ttyUSB0
[ 1942.312628] usbserial_generic 1-3:1.1: generic converter detected
[ 1942.312803] usb 1-3: generic converter now attached to ttyUSB1
[ 1942.312984] usbserial_generic 1-3:1.2: generic converter detected
[ 1942.313158] usb 1-3: generic converter now attached to ttyUSB2
[ 1942.313342] usbserial_generic 1-3:1.3: generic converter detected
[ 1942.313517] usb 1-3: generic converter now attached to ttyUSB3
[ 1943.225921] USB Serial support registered for GSM modem (1-port)
[ 1943.230129] usbcore: registered new interface driver option
[ 1943.230140] option: v0.7.2:USB Driver for GSM modems

Log File Locations, NetworkManager (Kubuntu 11.10):
$ grep -i networkmanager /var/log/syslog

NetworkManager logs
$ NetworkManager --version
Failed to read classid file: Object not found
0.9.1.90
[question: why the "Failed to read" message above?]

Configured NetworkManager to log:
$ sudo dbus-send --system --print-reply --dest=org.freedesktop.NetworkManager /org/freedesktop/NetworkManager org.freedesktop.NetworkManager.SetLogging string:"debug" string:""

Stop NetworkManager:
$ sudo service network-manager stop

$ killall -TERM modem-manager
modem-manager: nessun processo trovato

$ modem-manager --debug
modem-manager[2356]: <info>  [1324073336.223826] [main.c:167] main(): ModemManager (version 0.5) starting...
modem-manager[2356]: <warn>  [1324073336.246828] [main.c:90] create_dbus_proxy(): Could not acquire the org.freedesktop.ModemManager service.
  Message: 'Connection ":1.50" is not allowed to own the service "org.freedesktop.ModemManager" due to security policies in the configuration file'

start NetworkManager again:
$ sudo service network-manager start
network-manager start/running, process 2371

$ NM_PPP_DEBUG=1 /usr/sbin/NetworkManager --no-daemon
Failed to read classid file: Object not found
You must be root to run NetworkManager!

$ sudo NM_PPP_DEBUG=1 /usr/sbin/NetworkManager --no-daemon
Failed to read classid file: Object not found
NetworkManager is already running (pid 2371)

$ sudo service network-manager stop
network-manager stop/waiting

$ sudo NM_PPP_DEBUG=1 /usr/sbin/NetworkManager --no-daemon
Failed to read classid file: Object not found
... 
to be continued in:
$ grep -i networkmanager /var/log/syslog > ~/Chiavette/NetworkManager.log 
(in attachment)

$ sudo service network-manager start
network-manager start/running, process 2553

$ sudo dbus-send --system --print-reply --dest=org.freedesktop.NetworkManager /org/freedesktop/NetworkManager org.freedesktop.NetworkManager.SetLogging string:"info" string:""
method return sender=:1.61 -> dest=:1.73 reply_serial=2

NetworkManager.log switched off with:
sudo dbus-send --system --print-reply --dest=org.freedesktop.NetworkManager /org/freedesktop/NetworkManager org.freedesktop.NetworkManager.SetLogging string:"info" string:""
