I use triple.fi 10s as headphones. You can have any wired headphone plugged into the n900 and still have telephone functionality without a headphone mic. The audio for a phone call will still be picked up by the phone mic and you can hear the call on your headphones. You just need to answer/make/end calls through the phone like you would without the headset.

I also have the nokia bh-214 bluetooth adapter with built-in mic (you can plug any 3.5mm headphones into this adapter, i plug in my triple.fis). I normally only use this though when I commute so I can listen to my music and scroll through tracks without having to take my n900 out of my pocket. It adds a little hiss in the background to the audio but this is easily blocked out by turning up the volume a notch. When I use this on a call the quality and functionality is fine (you still can only answer/end a call with this, you need to make a call through the n900).