Messenger thinks any (or most) video sources are a "webcam".
For me, it is my TV tuner card -- not only would Messenger tell you I have a webcam (which I don't), but if we were to start a video conversation, you would see whatever is shown on the last TV channel I watched.
Messenger also recognizes another video capture device on my ATI Radeon graphics card -- which I seem to remember reading the card has, although I have never used it.
There is an option to not show presence of "webcams" to my contacts, which I have since then turned on, to keep people from bugging me about it.
