Don't forget about Sprint. You can get a standard voice plan and add data, or Simply Everything for unlimited everything (including 3g) $99/month. They have some slightly cheaper data-only plans or plans with unlimited data but not voice, too. Of course Sprint is CDMA (EV-DO Rev. A), though...

I believe Sprint claims it costs $15/month to tether. Not sure... I tether my N810 without paying extra, though, and no problems yet.