You can use PDANet to automate the setup of your phone as a modem if you want, avoiding the per-minute charges and the surcharge for tethering. They have software for both Windows and Palm devices. I used it with my Motorola Q from VZW for a while and it was bascically the same performance as what I would get browsing the web from my phone, but a much bigger screen and keyboard

If you have a BB already you can add on the tethering to your data plan for $5-15/month, depending on the carrier and it is a pretty good deal as long as you don't need to use the phone and data connection at the same time.

The AirCards cost more as an extra device (~$60/month generally for unlimited access) but you can also talk on the phone at the same time, which is nice.

For the NE Corridor on Acela I found that Sprint was very reliable for performance and connectivity, VZW was a close second and ATT/Cingular was a somewhat distant third.