Today I have been trying to create my own headset adapter cable. As already mentioned in this thread, the Bose mobile headset doesn't work on the N900. The pins 3 and 4 are swapped compared to nokia headsets.
Simply swapping the pins doesn't work! I suspect that more is needed, like a capacitor of some size or maybe more electronics. I have now created an adapter which only makes use of the audio (using a 3-pins 3.5" jack).
I hope that some manufacturer is going to create a working headset adapter. But for now, I'm satisfied with good audio only!

Some other (useful?) information:
- My WH-205 (N900) headset works fine, is always recognized
- And another headset HS-48, isn't recognized by the N900. No headset/headphone symbol is shown, but audio is routed to the headset!