According to two posts I found in the forums, a headset designed for the iPhone isn't perfect for the N800 since the posters had to pull the plug out by about 1mm to get the sound to work properly. To me this indicates that the plug for the iPhone headset is intended to be pushed in farther than the standard N800 headset plug.

Having said that, if you can tolerate this quirk, you should be able to use other mic/headset adapters like the Monster iSoniTalk Microphone Headset Adapter for iPhone or the Shure Music Phone Adapter for iPhone (both found at the Apple Store for iPhone Headsets.) There is also a Shure model of the Music Phone Adapter for Palm Treo, but I don't know if that would be better suited to the N800 or not.

Having said all that, I don't own any of these products so I have no direct experience with them. I would prefer NOT to have to worry about how far the plug is pushed in to make my headphones (and microphone) work on the N800. I guess I'll stick with the built-in microphone until somebody finds something that fits the N800 perfectly.