This behavior is actually determined by the operating system, not iCamSource,
With Snow Leopard (10.6), the operating system only allows access to the camera (built-in and USB) when the account is active. If you switch to another account, the program will not have access to the camera. I know that one person has gotten it to work by having an instance of iCamSource running in each account, but that won't work at the log-in screen.
With Leopard (10.5) or Tiger (10.4), the operating system does allow access to the camera when the account is inactive. In that case, if you are switching between accounts, only one of the accounts can be running iCamSource, otherwise they would conflict with each other.
