A lot of dogs do that redkitty. I read that it could be because catfood has more protein in it, thus making the "end" product appealing to them. My dog was eating his own (also not totally uncommon) but I don't think he has since I accidentally started forgetting to give him his brewer's yeast (for fleas). The funny thing is, natural remedies to stop that contain brewer's yeast because it is bitter and they say dogs don't like it. Cubbie loves brewer's yeast!
My mom's dog Baby would eat anything my mom would eat, except lettuce. She was crazy about Nabisco ginger snaps though.
We had a cat that had come from an abusive situation. When we first got her she would eat so fast that she would get sick. She finally stopped doing that, but if one of the other cats came near her while she was eating she would eat it as quickly as possible. I think she would have eaten rocks if she thought the other cats wanted them!