My cat does this and doesn't even bite when I rub her belly gently, so I guess I'm either lucky or my cat doesn't conform to normality!

Something I've learned over the years ( and five cats) is: Never touch a cat when he/she is taking a sunbath. It's a trap!

I live with two cats who both think that you rubbing their bellies while they're sleeping is only their natural due, and they often protest when you stop.

Agreed, that part of the article is simply wrong. My neighbor's big fat fluffy cat comes over whenever it sees me outside and immediately flops over on its back and starts rolling around. It seems to luxuriate in having its belly rubbed, and crave touch in general. Flop and roll, belly rub, get up, rub something, flop and roll, belly rub, over and over. Eventually it's had enough, goes over and scratches a tree, then waddles off.