I have a keyboard now and can describe in a bit more detail. Up until I installed Snow Leopard, I could start 3 instances of icamsource (one built-in ISight, and two forwarding network based webcams). I could then go to the Login window - not actually logged out though, and the video/audio would continue to work.
With SL, as soon as I click Login Window... the green light on the built-in iSight goes out. The icam client on my ipod touch continues to see the webcams, but the built-in iSight if offline. /var/log/system.log shows:
Sep 30 13:37:52 Bigmac-4 /System/Library/CoreServices/loginwindow.app/Contents/MacOS/loginwindow[264]: Login Window Application Started
Sep 30 13:37:55 Bigmac-4 loginwindow[264]: Login Window Started Security Agent
Sep 30 13:37:57 Bigmac-4 UserEventAgent[273]: Error loading /Library/Audio/Plug-Ins/HAL/DVCPROHDAudio.plugin/Contents/MacOS/DVCPROHDAudio: dlopen(/Library/Audio/Plug-Ins/HAL/DVCPROHDAudio.plugin/Contents/MacOS/DVCPROHDAudio, 262): no suitable image found. Did find:\n /Library/Audio/Plug-Ins/HAL/DVCPROHDAudio.plugin/Contents/MacOS/DVCPROHDAudio: no matching architecture in universal wrapper
Sep 30 13:37:57 Bigmac-4 UserEventAgent[273]: Cannot find function pointer NewPlugIn for factory C5A4CE5B-0BB8-11D8-9D75-0003939615B6 in CFBundle/CFPlugIn 0x1002093a0 </Library/Audio/Plug-Ins/HAL/DVCPROHDAudio.plugin> (bundle, not loaded)
Sep 30 13:38:05 Bigmac-4 SecurityAgent[271]: NSSecureTextFieldCell detected a field editor ((null)) that is not a NSTextView subclass designed to work with the cell. Ignoring...
I'm guessing the Login Window Security Agent now disables the built-in cameras when you're not actively owning the display.