This behaviour has got much worse recently. He's a happy little boy, and none of this seems to be done out of frustration or annoyance. He's had a habit for a long time now of grabbing other children, which I've always delt with by saying "no" firmly, explaining why it's bad, and making lots of fuss of the child he grabbed. But then a lot of my friends children have gone through this stage too so I wasn't too concerned.
In the last month or so he started to grab mine and DH's face really hard pretty regularly. He's always smiling when he does it, and it often follows a "kiss" that he's also started to do recently. He grabs my boobs too (I'm still BF and he'll do it when he' s finished), and this morning he grabbed and pinched my forearms really hard. Again, smiling all the time.
At nursery they've been addressing the grabbing other children in the same way we have at homje, which is good.
At the moment, we're saying "no, that hurts" very firmlyand taking his hand away (although I occasionally yelp as it can be really painful). We then take his hand, open it out, and get him to stroke gently where he's just grabbed. I tell him to "be gentle like you are with a pussy cat", as we're taught him to be gentlw with dogs and cats. So far, it's not working at all 
Any ideas gratefully recieved......