I assume you already know about the iGo (which I so far like).

The only two others which seem reasonable are the Palm keyboard:

http://www.keyboardgalaxy.com/showroom/984.cfm

This is probably closest to the Newton, but is not verified to work so far (anyone have one?), but I doubt there would be a problem.

The new Apple wireless keyboard is bluetooth. Also not verified to my knowledge, but it is a real keyboard.

I have the old (big full sized) apple keyboard, and it is not stable with the n810, but works fine with my apple computers.