I have given up having several USB cameras on one computer, iCam, or the OSX, mixes them up so it won't work stable. I've tried four different MacMini and iMac's. Even bought USB'cams from different wendors to get different names on them on the USB bus. Works perfect with network cameras, but not more than one USB-cam. Can have one USB cam on each computer though, no problems getting several computers to use the same iCam-account.
By the way, an interesting feature would be noise-detection, as well as movement. I want to hear my dog barking...