I once used 2 "Logitech quickcams for notebook pro" with a 10m - usb repeater and a 3m extension. It worked fine. I think the maximum cable-length for USB 2.0 is 5m. Exeeding this limit can just be done by using repeaters.
There are not many USB 2.0 Cams on the marked yet, but as I guess you are using a mac have a look for long fiewire-cables. They are working with my external Disk - so a webcam won't be a problem.
If you are using a PC make sue, that you really get a USB2 cam - with all usb 1.1 cam, compression is horrible - and your patch pretty slow.