I would like to use my Canon G11 as a webcam, and I thought this would be as simple as getting a video capture card to convert the composite video out of the camera into USB on the laptop. I have tried a number of capture cards now of which show up as a webcam. They allow me to capture video from the device fine, but that's not really what I want to do.
It is going to depend on what software you are using and what it expects for a webcam. In theory, if the capture device supports Window Video Capture, it should allow for you to rig it up as a webcam in any software that will work with a Windows Video Capture device, but not all capture devices and not all software that uses a webcam support Windows Video Capture (or similar technologies on other platforms) to allow for the connection to occur.
Sometimes it is possible to use third party software to make the connection too, but that depends entirely on the capture card and client software being used.