I shoot primarily weddings and always bounce, usually at half or full power. I used to over heat my units all the time. Before the firmware upgrades, I drilled two little holes on the side of the unit, where the diffuser clips in, and then used a baby aspirator to blow air into the unit (if I had the option of dumping the batteries and leaving the door open, I would do that as well).
Just drilling the holes made a noticeable difference. With the aspirator, I could cool a unit to operational temps (pre-firmware) in a matter of minutes. With fresh or iced batteries I could keep shooting for 30 minutes or so.
My advice is to have a backup flash. If that isn't possible switch to normal alkaline batteries leading up to important events. The normal batteries don't have the ability to push so much current and therefor heat into the unit. I have only had one sb900 over heat while using normal alkalines. If you're like me, the cost of the extra batteries is too expensive to justify them so modify your shooting technique.
Shoot at higher ISOs. That lowers the power that your unit has to put out and thus lowers the amount of energy that it needs to take in. At receptions, when I know I'll be shooting a lot, I never shoot lower that 800 iso. Shoot at wider apertures. I can shoot on one set of my rechargeables almost all night at 800 iso and f/2.8 in a room with 10ft ceilings.
It's a great unit. That said, I always carry backup because it fails so often. I will say that it has some 50k fires, mostly near full power and is still going strong. Then again, so does the sb800 and it has never overheated on me.