I have Match and I hear an iAd everyone once in awhile. They aren't very long or obtrusive like the Pandora ads, but I hope the ads stop when it goes public.
Update 2013-Aug-09: I've been listening to iTunes Radio for over 2 straight hrs and just received my first Ad. I'm not even sure it was an ad, it was about a 2 sec bleep. I think it's getting better and learning over time.