A modern Psion Series 7 would be nice as long as it majors on a long battery life and being able to be charged from fairly low output USB (0.5A) so you could charge it from solar/bicycle.

The main problem I had with the n900 was it wasn't big enough and the battery life was shocking for a portable computer. I also thought it was rubbish as a phone compared to the n9 but you pitch your compromises in different places.