I'll admit it: I loathe Bluetooth headsets. It's impossible to walk around with one of those things protruding from your head without looking like a total tool. No one's impressed, Lieutenant Uhura.
I'd like to think it was that reason that prompted Apple to stop selling the iPhone Bluetooth headsets, but it was probably poor sales. We don't have sales figures, of course, but I've never seen one in the wild.*
It's also possible that Apple is up to something else. You'll remember that iPhone OS 3.0 touts support for A2DP Stereo Bluetooth connectivity among its features. The headset which shipped with the 1st generation iPhone had a built-in switch, similar to the one included with the current iPod shuffle. Perhaps a future model will control the iPod and answer calls over Bluetooth? If so, it had better be pretty cool.
*Out of fairness, I'll state that I live in a town of 4,000. Not exactly a hive of early adopters.