I wouldn't be so sure. Nikon uses the same sensor in the D90 as in the D300 which is excellent in low light environments. The Canon's footage looks good too, you'd probably be happy with either one.
Back to the original topic, I don't think a D90 would work because:
- I don't think there is a driver that can make is a USB webcam. I could be wrong though. There is a Nikon SDK that may enable this functionality
- The bandwidth requirements would be huge to stream 720p video live across the Internet.