T-Mo TOS don't appear to allow tethering to 3G. They do allow tethering on Edge at no additional charge (so back down your device.) Their happy helpful customer support (really, I think they pump drugs into their air) will happily walk you through tethering on a BlackBerry, no additional cost. Tho >10GB in a billing cycle you get throttled.
That said T-Mo plans are cheapest, win JD Powers for service, are great about supporting random phones, coverage has improved considerably in the past few years, and T-Mo does lead on Android.
Same for G1 users.
T-Mobile offers free unlimited access to all of their T-Mobile hotspots (which includes the new AT&T rebranded ones for another 5 years) to their G1 data plan subscribers. T-Mo even has a free GPS-enabled Hotspot Finder for the G1 that handles the login details for you.
I recently got asked why I'd ranked another