I've just got this problem yesterday and can't manage to find a solution. This morning, I and the technician of Nokia Care here try lots of headset and find out that only the headset that came with Nokia E63 works with my N900, but the sound quality is terrible.

I don't know what happen because the phone always recognize that there is a headset plugged in (dmesg report) but the sound always come out from the speaker, not the headset.

Confusing.

P.S: It's very strange that although there is no sound come out from the headset, the music player stil stop playing whenever I take out the jack.

Is there anybody has the same problem with me?