Saw this post was active recently so I thought I'll reply

Recently dug out my old N900 to play with and found the same headset issues with my in-ear soundmagic e10's.

I live in China and found these 3.5 jack to 3.5 jack adaptors on a web shopping site (tmall). I'm not sure of the wiring and my chinese is not that great but they're cheap (8RMB) so I've just bought one to try out. I'll let you know if they work when they arrive though I'm not sure where to buy them in US or Europe.

http://detail.tmall.com/item.htm?id=...&cm_id=&pm_id=