I picked one up on eBay [0] that is currently in transit to my apartment. Manual focus, somewhat garish looking and with probably crummy resolution, but I'm willing to try it for $4.

There are a large number of similar cameras on the market that ship from China/Hong Kong on eBay. YMMV, but if you've got a few bucks and are willing to wait, it may be worthwhile (depending on your use).

[0] http://www.ebay.com/itm/USB-50M-6-LED-Night-Vision-Webcam-Ca...

Indeed. (I have a project that needs to distinguish between cats in the dark. No kidding!)

A pet door?

Cat-specific food bowls :-)

I was researching this a few months ago, but gave up upon the cat facial recognition. The best I could find for the raspberry pi was opencv haar filters for humans, and that was just detecting if a face was present in the video frame. I'm curious, how are you planning on doing cat facial detection for this?

maybe he has a white cat and a black cat... but I would probably use RFID chips in their collar.

That's what I'm trying first. RFID range is an issue, however. The electronics for this is easy, the problem is that I suck at carpentry.

The cats in question are pretty different (one black, one orange), so discrimination in visible light is easy. Not sure about IR yet.