I use the iGo Stowaway Ultra-Slim BT Keyboard with my N800. I mostly using it for long emails when I'm on the road and entering data into gnumeric. It's a bit of a pain to carry around the KB for day to day use (it's larger than the N800), but for road trips where I know I won't have a laptop/desktop around, it's indispensable.

Also, it's very handy when working in Xterm and the ocassional abiword/note document.