There are a few projects out there [0][1] that make it fairly simple to set up a motion detection capable solution with a simple USB webcam. I'm currently waiting on a cheap infrared camera from China ($4!) for use with my Pi, but I've done testing with an older Logitech HD camera and have had excellent results.
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).
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?