My dog use to do that too, but she would do it on her back. If you stopped rubbing her belly she would start swatting at you, if you still didn't do anything then she would get up and shove her face into your hand and put it on top of her head. Then once you started petting the top of her head, she would flop back down on her back for more belly rub.. she would do this for hours..
I miss her =(
here she is biting- swatting me and making me lol..
[video=youtube;6zm1DEHVdfo]http://www.youtube.com/watch?v=6zm1DEHVdfo[/video]