If you want something newer than an N810 ...

then the three things that come to my mind are:

1) Sharp Netwalker ($600ish) ... runs Ubuntu
2) UMID mbook M1 ($450ish) ... runs Windows
3) Samsung Mondi (?) ... runs Windows Mobile

Though, you can probably get a T-Mobile G1 for cheaper than any of those. Depends on whether or not its browser will do the job for you (and how you want to deal with the SIM card ... I'm not sure if you can really go wifi-only with it).

Keyboardless gives you a lot more options ... like the Q5 and Q7. The Q7 can be had for under $200, and runs Ubuntu.