Yep, Foxy does it all the time. I'm sure someone told me its what they do when they want to...eh...do you know what!!!! Foxy's belly is so baby soft though, I cant resisit giving it a good scratch for her!!!
It can be for submission, say if you've just told your dog off but it can also indicate they want their tummy rubbed. My dog doesn't like getting her tummy rubbed but she will show her stomach submissely. She was stitched up badly after being spayed so her wee tummy is v.sensitive.
My uncles gundog loves getting her tummy rubbed though! She rolls right over but likes to lie on your feet at the same time!