If its not getting good electrical contact, flashing the firmware won't make a difference.

I still haven't heard if you've tried other headsets. Did you?

Have you shut down the phone, removed the battery, waited a bit, and then reinserted battery/rebooted? Might as well give that a go first.

The jack on these phones is like every other one I've seen - it has a small switch inside that is depressed when the headset is inserted. That switch is what tells the phone to enable/disable the port. If the switch is bad, loaded with lint, or has otherwise failed, it won't work. You'd need to get the jack fixed. Usually the failure mode is "stuck on" not "stuck off", so its a little unusual - but not impossible.

Take a look in the hole, does everything seem ok? You should see *two* copper contacts. One on the top, one on the right, when looking "side on" at the n900 face up. If you only see one contact, then it's broken for sure.