I once met a cat like that. It used to sidle up to the guest (me, boyfriend of the female owner), purr, rub my leg and generally show all manner of interest and affection before rolling languidly onto its back for a belly rub. The uninitiated would then rub her belly. I knew better after making that mistake exactly once. There was blood involved, and it wasn't the cat's.
Weird cat. The other thing it did was snuggle up to the guest on the couch, working its way higher and higher until it could suck an earlobe.
As I mentioned, weird, and you did definitely not want to do the belly rub.