When cats are lying on their backs they are in a defensive position – they are NOT asking for a belly rub. In this position they have use of all their weapons: 4 claws and teeth. Leave your cat alone when he does this.
Yes -- this was in the last post, but it's important enough to state another time. A lot of people get bit or scratched when they mistakenly think the cat is asking for a belly rub.
Below is a diagram on cat body language which illustrates a little better how to read your cat's mood. Now go grab that fishing pole toy and have fun with your feline friend!