Every time I post something to my staff on advertising for my forum, I always advise them against PM advertising. I tell them that not only will people be more likely to ignore the PM or report it to the admin, it also ruins my board's reputation. And I don't PM advertise either. I've had people PM advertise on ME, and I usually ignore the PM.
PM advertising is the worst. As an admin, I don't trust people that join a forum simply to pm advertise. That's dishonest. The only advertising I allow on my forum are links in the signature line. And I do have affiliate links on the portal page. But posting "30 second" commercials for other forums--a big no-no in my book.
One person joined my board, and all he did was PM advertise me the one time to talk about another advertising board. Now while I got another board to advertise on, I ignored the PM altogether. This person wasn't even kind enough to post one article before the PM. Go figure.
this is a bit different, but I am a member of my friends site and shes a member of mine and I recently asked her if I could PM one member of her site and invite them to come and join my forums.
my friend said this was ok and I pmed the person in question invited them but said that I had asked admin (my friend) before doing it, so she knew I wasn't taking advantage
Personally the original idea while if the forum owners say yes i suspect it would simply look spammy to there users - i actually disallow it on my site where i allow them within the forums themselves.