I'm having the same problem. For me it begins whenever I call a Verizon landline phone and goes away when I re-flash the phone. When the problem is present I can still use the headset (WH-205) as headphones by inserting the plug into the N900 very,very slowly (and most often I have to do so a large number of times before it works) but the microphone does not work and telephone calls go through the phone as they would without a headset plugged in (even when the headset is plugged in). This means that in the car I cannot use a wired headset as a handsfree device and that means in my state of Massachusetts it would be illegal to use my N900 while driving (I have lost numerous bluetooth headsets and am tired of paying for them so they're right out).
For safety sake it is a problem which should be fixed.