i can confirm this bug on the same version ubuntu gnome. This didn't work since the early alpha so the problem might lie on the first commits of the gnome 2.26 development (2.25.x).
if you use ALT+F2 it's another way to get it to work correctly so the only part broken is the entire menu system of the panel which opens windows on the first screen ALWAYS it looks like the screen enviroment variable is wrongly set somewhere and i can see there are some commits which changed the way they are launched like http://git.gnome.org/cgit/gnome-panel/commit/?id=b5e5fe98ef2aae4c637a9b6716bc579e21b502d9 which could be worth looking although i didn't have the time to check it out.
Tried various beta of nvidia drivers in the 185.x series and i really doubt it's an nvidia problem as launching manually fixes it (from terminal passing the right screen enviroment variable or using alt+f2 launch from the second monitor)

this is my xorg conf no xinerama nor twinview so two panels launched one per screen, the thing was tested from a live so it's a clean system.

Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0" 0 0
    Screen      1  "Screen1" RightOf "Screen0"
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
EndSection

Section "Module"
    Load           "dbe"
    Load           "extmod"
    Load           "type1"
    Load           "freetype"
    Load           "glx"
EndSection

Section "ServerFlags"
    Option         "Xinerama" "0"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/psaux"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Keyboard0"
    Driver         "kbd"
    Option         "XkbRules" "xorg"
    Option         "XkbModel" "pc105"
    Option         "XkbLayout" "it"
    Option         "XkbOptions" "lv3:ralt_switch"
EndSection

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "AUO"
    HorizSync       30.0 - 75.0
    VertRefresh     60.0
    Option         "DPMS"
EndSection

Section "Monitor"
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "CRT-0"
    HorizSync       31.0 - 68.0
    VertRefresh     60.0 - 85.0
# HorizSync 15.625# 30.0 - 60.0 #30.0 - 50.0
# VertRefresh 50.0 # - 75.1 # 50
#HorizSync 15-17
#VertRefresh 49.0-61.0
# 702x576 @ 25Hz, 15.6kHz hsync, itu-r bt.470 PAL, analog broadcast
#ModeLine "702x576@25i" 13.5 702 722 785 864 576 581 586 625 interlace -hsync -vsync
#Modeline "640x480@3i" 0.53 640 672 672 704 480 492 493 505 interlace
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    Option         "Coolbits" "1"
#    Option         "CustomEDID" "CRT-0:/etc/X11/edid.bin"
    Option         "TVStandard" "PAL-B"
    Option         "TVOutFormat" "SVIDEO"
EndSection

Section "Device"
    Identifier     "Videocard0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 8600M GT"
    BusID          "PCI:1:0:0"
    Screen          0
    Option         "Coolbits" "1"
EndSection

Section "Device"
    Identifier     "Videocard1"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 8600M GT"
    BusID          "PCI:1:0:0"
    Screen          1
    Option         "Coolbits" "1"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Videocard0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "TwinView" "0"
    Option         "metamodes" "DFP: nvidia-auto-select +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Section "Screen"
    Identifier     "Screen1"
    Device         "Videocard1"
    Monitor        "Monitor1"
    DefaultDepth    24
    Option         "TwinView" "0"
    Option         "TwinViewXineramaInfoOrder" "CRT-0"
    Option         "metamodes" "CRT: nvidia-auto-select +0+0"
EndSection

Section "Extensions"
    Option         "Composite" "Disable"
EndSection
 

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*** This bug has been marked as a duplicate of 580103 ***