@Adamsq. I'm to looking for the exact same thing!!
What has been for a long time standard, easy to use, wide support - streaming video over FireWire - has now become a luxurious feature for the more expensive cameras, it seems.
We're left with no standard way of streaming video over USB. Some of the SSD camcorders offer a "web cam mode", where they go into a baby mode with minimal settings, and a low resolution feed going through USB, camera identifying itself as a standard webcam.
My trustworthy old DV camera broke a couple of weeks ago. Now I'm stuck with an automatic web camera (not a real alternative since I need manual camera settings). Or a cheaper DV camera that does only record Interlaced (not a real alternative since I'm using it in a live video to screen it needs to shoot progressive. Realtime de-interlace looks horrible.) Or an awesome Canon EOS 7D that does not offer live-video without interface graphics blocking the screen through video output, or with Canon's live-view not identifying itself as a generic video source... It streams, but you cannot catch the stream. Now how stupid is that? On PC you could use http://www.extrawebcam.com/ to capture the DSLR live-view stream, but no luck on Mac as of now..
Now I'm thinking maybe for Live gigs perhaps a camera connected to a SDvideo-to-firewire converter would do it.. But it has to offer manual controls.
Awwhhhgghh...
Please let me know if you find any decent solution.