"Really? A fatal flaw? I think it's a bulb saving feature."
Well, with the increase in Strobist type use and abuse of portable speedlights, and the new breed of digital wedding shooters who turn in 1,400 to 1,600 shot count weddings, SB-900 units are overheating and locking up with enough frequency that the older,discontinued SB 800 has become the preferred flash of many people who want to fire a speedlight off of an external battery pack like a machine gun.
It sounds like Nikon might have gone a bit overboard on the thermal protection feature of the SB 900, but then again, the new trend of using one or two speedlight flashes instead of a 200 watt-second studio flash means that worldwide, hundreds of thousands of speedlights are being plopped onto umbrella mount brackets, and people are doing their "Strobist" stuff with flashes and many of them are/were deliberately ignoring the manual's stated flash number/cool-down time and just burning up flash tubes on SB 600 and SB 800 flashes.
So, yeah, the bulb-saving thermal protection the SB 900 has is good for SB 900 flash tube life span, and good for Nikon warranty claims reduction, but it seems that there are MANY wedding photogs and Strobist people who'd rather have an unprotected SB 800 that they can fire until a meltdown occurs--and they want it so badly that they have driven the price of the remaining SB 800 new and used inventory up to ridiculous levels.
It's almost like low-flow toilets and old-style toilets--I had a contractor tell me people call him all the time offering 500, 600 for an "old style toilet", and not the new 1.8 litre per flush models. Many people don't want any limits on any area of their lives. In-town speed limits of 40 MPH--PShaw, people are driving 50, 55 MPH down my neighborhood throughfair, and the motorcycle cops are writing $247 speeding tickets on average two days a week, but it doesn't stop these Suburban moms and dads from hauling A$$ through the neighborhood in their big gas-guzzling SUV's.