A reader writes on SpitCam:

I found a program which lets you use a prerecorded video and stream it as your webcam. The product is called SplitCam at SplitCamera.com. I was able to share a video with my friends in realtime. SplitCam shows up as a webcam option. It even allows you to use images instead of videos so you can share them in realtime. Why this is great is you don't need to email, transfer, or upload the file. You could stream it instead. This could even be used as a way to have a video answering machine if there was some way to make skype auto answer the call.

SplitCamera is a freeware virtual video clone and video capture driver for connecting several applications to a single video capture source. Usually, if you have a web-camera connected to your computer, you cannot use it in more than one application at the same time, and there is no standard Windows options that makes it possible. SplitCam driver allows you to easily multiply your web-camera video in any conferensing software like ICQ, Yahoo, MSN Messenger, or whatever... and to broadcast it to many users at a time. With SplitCam you can connect up to 64 clients to a single video source. In a few words: SplitCam does just what its name says: it splits the video stream coming from the video source and tunnels it to numerous other client applications.

Self explanatory really. I'll have to find some time to try it out.