The battery life (in offline-mode) and instant-on of the Nokia 770 would make it a great PDA replacement, if good software was available.

My 770 currently serves as
- a surfing pad (the screen is small, but excellent for news sites; shopping on Amazon is also perfectly possible and fun with it; optimized 150% zooming gives the best results for well designed websites)
- a radio
- a RSS reader
- a development platform
- a PDA replacement for taking small notes and making sketches for which i used to waste paper and lose them after a short time

And when Doom will be released, I might also use it as a game console.