You know Twinkley... they say that a cat is still 95% instincts. They just do things that seem odd, because they are basically instinctual... they don't even realize it themselves.
Anyway, as part of their instinct... they feel very very vulnerable on their back. They feel completely defenceless with their stomach exposed.
This is why you seldom see a cat on their back... it's believed that a cat has to be 100% comfortable with their surroundings, and the people in the room with them, before they'll willingly lie on their back like that.
So it's a good sign... it's a big big sign of affection and trust from your cat when they lie on their back.
