My thoughts return to the PSP with its serial inline remote for controlling media playback. Though I haven't yet driven with my N800, I can imagine myself wanting to use something tactile while keeping it docked, and not have to try to touch the right spot on the screen, so I can keep my eyes on the road.

I would imagine there are many apps that could benefit from extra hardware buttons as well.

I haven't seen any mention of such an accessory, so I'm asking you all if something like this exists, and if not, how much work it would be to create one.