In the 1.5 years I've been a member, I've never gotten a nasty PM here (though I did get quite a few horrible emails from some anonymous chazzer, shortly after I made my email address public)
I don't send PMs all that often, but when I do send one that could be percieved as "nasty" it is to ask someone about something that I don't want to be discussing in the public forum. Sometimes I suppose it appears confrontational but I ask through PM so they don't have to show the entire forum what they have to say. If I'm debating with somebody and the thread dies, we take it to PM.
If someone were ever to PM me just to attack me, they'd be ignored. End of problem