Currently using two different ones:

Nokia BH-504 Stereo Bluetooth Headphones.
Plantronics Voyager 520 Bluetooth Headset.

One point of note: the Voyager 520 bluetooth headset has multi-point (which means active connection to two phones at one) and the N900 seems to perform fine whilst my Xperia X1/N97 used to randomly disconnect.

I'd recommend either... the 504s for music playback & the occasional phone call or the Voyager 520 for those who carry more than one phone and/or use the phone a lot whilst driving (battery is AWESOME!)