For rickles: if you're looking primarily for an email appliance, wouldn't you want something with some sort of keyboard?

For the rest of you nerdlings who don't know what you're talking about:

Cingular 3g is HSDPA which gives you 3.6 Mbit/s peak download spead. Sprint and Verizon 3g are EVDO (2.5 Mbit/s) and EVDO Rev. A (3.1 Mbit/s) in certain places. That's pretty academic though, those are just the peak speeds, and both are very bursty transmission methods, so most of the time you're not going to be running at peak speed. It really just comes down to where you can get better signal strength in more places.

As for the N800, cellular networks don't matter because it doesn't have a cellular radio. It has WiFi and Bluetooth. You can connect to a WiFi access point if you're near one, otherwise you need a cell phone that is capable of acting like an aircard and sharing the connection over Bluetooth.