I was having the same exact dilemma as you last month. My goal was to wait for the 2010 MY. I waited a couple weeks, but I couldn't take it anymore. I lost my patience and ordered a 2009 m3 sedan, and I got an awesome deal on it too.
First off, the 2010 m3 sedan will barely have any changes at all since the '09 version already had a significant update with the tail lights and iDrive, and some other stuff. Most people on this forum would agree that the 2010 will have very small changes, if any at all. Also, if I waited for the 2010 I'd have to wait to place my order some time in September, and I would get my m3 in November....If not a little later. My actual order was processed on May 20th, and my m3 is scheduled to arrive in about 3 weeks.
Honestly, I probably didn't make the smartest decision by not waiting for the 2010.....Or maybe I did. What if the dealers don't discount the 2010's as much when ordering first starts? I mean it's gonna be a brand new model, so dealers are gonna be strict in the beginning I would think. A lot of questions went through my mind, but I just went with my gut feeling. What matters most is if you're going to be completely happy with your decision.....And I am.
My advice is to wait for the 2010 MY. That would be the smartest move at this time. If you don't have the patience like me, then go with your gut feeling. All the best with your decision!