In Hobbes, however, well....
It isn't that he's acting out of malice, after ten or so years of having him about I honestly don't believe it's in his nature to be malicious, but he does take it as a call to action, or play pretty much every time you stroke him there, and his favourite game sees him rolling about on the floor trying to catch hands that are attempting to scritch his belly. Given how sharp his claws are, this may be taken as a bizarre form of masochism on the part of the humans involved (usually me and/or my friend Emma). You can find a video of this on my Facebook page.
In the meantime, I found this cutie on Youtube, demonstrating exactly what I mean by the trap (though the fact that she looks a bit 'hey what are you doing, I'm toasting myself here' does rather undermine the idea that she's a savage beast out to tear hands up).
It should be said that not every cat who dislikes the tummy rub is going to be violent or treat it as a start of playing. My parents' last cat, Catkins, didn't really like having the belly fuss and would gently push any hands away. She would be incredibly patient about it.
And for me, at least, the fact that the cat's belly is so often a trap is actually part of their charm and I'm not above going in for a fuss to just get Hobbes to wriggle about and trying to catch me.