It appears there's a possible regression in network-manager 0.8-rc2 with regards to the handling of a Sierra 3G modem.

The original bug report can be found here:

https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/509738

From the original bug:

On Karmic network-manager and modem-manager from the PPA worked fine with a Sierra 3G card ( USB ID 1199:6813 ). On Lucid it seems that the PIN is not submitted.

The bug seems to be a regression in either modem-manager or networkmanager.

Karmic working version:
network-manager: Version: 0.8~rc1-0ubuntu1~nmt1~karmic
modemmanager: Version: 0.2.git.20100102t025215.a06b3f2-0ubuntu1~nmt1~karmic

Karmic broken version:
network-manager: 0.8~rc2-0ubuntu2~nmt4~karmic
modemmanager: 0.3-2~nmt1~karmic

STEPS to reproduce the problem:
Install latest network-manager and modemmanger from https://launchpad.net/~network-manager/+archive/trunk or latest available Lucid.
Select Edit Connections, add Mobile Broadband connection for D1, specify username and password and APN and PIN.
Connect to T-mobile (D1) connection 1
Applet icon starts spinning
Expected outcome: 3G connection established.
Actual outcome: 3G connection does not work. NM logs a warning in syslog that the PIN for the sim needs to be entered:
NetworkManager: <WARN> stage1_enable_done(): GSM modem enable failed: (32) SIM PIN required

The problem can be circumvented by using a sim card without PIN. Then nm/mm works just fine.