I had been wondering about this too, Jameson didn't use to do it when we first got him, but now every time I get back home and unlock the door I'll find a gorgeous kitty on the carpet, on his back, wiggling around, showing the belly, reaching out with his paws and purring like a tractor.
Infact if he's in the garden and I'll come outside he'll do it aswell. He loves rolling from side to side too when he does it, and moves his neck and head back and forward!
He doesn't mind his belly being stroked, but will let me know when enough is enough.