Does anyone know if there is an adapter that would allow me to use this Nokia/Sennheiser headset with the N800?

http://europe.nokia.com/A4400018

This headset uses a 2.5mm jack, while the N800 uses the 3.5mm jack, and I would like to seriously consider this option for my internet telephony

Casual search for a 3.5MM to 2.5MM (reducer) adapter yields

http://www.cell-phone-accessories.com/3-5mm-2-5mm.html

I think that is what you need and not the other way around

Actually, scratch that. I re-read your link and these adapters won't transmit the voice part from the mic, just the stereo audio. For that, check these google results