A German guy has done it ... the first USB webcam driver is now available, but only for the Philips ToUCam Pro. I hope that other cameras will follow soon as this is written as open source. Read about the 'wc' driver and its current status on the wc home page.
[Editor's note: I used Oculus for OS X over the holidays to drive our FireWire video camera as a web cam, and it worked perfectly. It captured an image every second to the hard drive, and uploaded an image every 90 seconds to our website for a couple of hours. The Oculus site lists some USB cameras as supported, but I have no idea if they work in OS X or not.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020111023936383