For the record, I've just faced the same problem with my stock n900 headphones. They were used for 10 times or so, so I don't think that the problem is with wiring. And they work with my laptop just fine. The problem occurred right after incoming phone call. I've accepted the call, then pulled them out to talk via phone, and then pulled them back in after the call was finished. That's pretty much it

Edit: Also I have a strange pulsating noise, like bad TV signal or something, when I plug in the handset very slowly.

Edit 2: Looks like very slow plugging in kinda fixes the problem with sound transmitting to the headphones, although n900 still doesn't show me handset icon. Stay tuned.