I've got some advice based on my first attempt. Here's my first design: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1282104243/0#0
The problems were:
It dispensed too much food at once.
There was some spilling (which would cause a jam in the mechanism)
So for my next version I would use a smaller opening so that less food comes through at a time. I would also have a smaller closed chute for the food to slide through.
Maybe I would have some rubber or cloth hanging down a half-inch so that there would be a seal, but no food could get jammed between two hard moving edges.
An alternative design would be to have a drawer that would slide forward and allow food to pour out the bottom.
I've also seen a design where the entire drum of food spun around and a bit of food came out of a hole each time.