Ha ha ha ha - it depends on the cat really, but probably for the majority of them, the belly is a trap and they don't want you to pet it. Showing it to you is a sign of trust, but they don't usually like people petting it.
That being said, some cats do - Maggie LOVED belly pets. She would have let me pet her belly all day.
Yuan generally doesn't like belly pets, BUT I have discovered that if I knead his belly like cats do with their paws, he's okay with that! :)