I think that's just because they've reused their normal phone headset and just added a new minijack connector. I would be very surprised if any input through the headset connector is accepted. When using a nokia headset connector (used for phones) there are more wires, one of which, I'm sure is for this button.

Good idea though. Another idea - inspired by recent reports on this forum would be to use your (Sony-Ericsson?) phone to remote control your N800. It seems it works right out of the box like on a normal PC. Great concept - I wish Nokia built this into their phones as well

// Kny