One thing to be aware of: you can get dirt-cheap data (at 19.2 kbps, alas) by going with a Motorola iDEN phone (for Nextel), and using a Boost pre-paid SIM.

The data usage (always-on 19.2 kbps) is free, but the SIM must be topped off with at least $20 every 90 days to keep it alive, even if you use no voice minutes.

Sounds like that kinda cheapness/slowness tradeoff may not be what you want, but everyone should know about it...

I'm gonna take the plunge and get such a setup shortly, as I have University WiFi most of the time, it'll be nice to fill in the gaps even at low speed.