I am having the same problem my Nokia WH-205 headset is working (tested on my n810). my n900 does not recognize it.

searched on the bugzilla and found this problem: https://bugs.maemo.org/show_bug.cgi?id=6360

I plugged in and removed my WH-205 headset that cam with the device twice,
I plugged in and removed a stereo headset twice.
I then ran "dmesg | tail" in x terminal and got this response.

~ $ dmesg | tail
[ 971.960632] headphone (GPIO 177) is now connected
[ 973.812164] headphone (GPIO 177) is now disconnected
[ 975.960632] headphone (GPIO 177) is now connected
[ 977.804290] headphone (GPIO 177) is now disconnected
[ 980.077697] headphone (GPIO 177) is now connected
[ 981.671600] headphone (GPIO 177) is now disconnected
[ 982.968200] headphone (GPIO 177) is now connected
[ 985.218383] headphone (GPIO 177) is now disconnected
~ $

The WH-205 headset did not display an icon nor would it play sound.

would love to get my n900 working again any help welcome
thanks,
Tim