"...Just a quick note on AT&T:
... Last I checked, they have a 'tether' package but I don't recall the rates..."
AT&T's tethering package costs 60.00 a month for 5 Gigs of date. If you have a blackberry, there is a program called tether (www.tether.com). It handles all the connection stuff for you and connects your PC to the net through you berry. It costs around 50.00, but is feequently on sale for half price. Once time charge, no monthly billing. My understanding is that it cloaks your tethering and is undetectable by the carrier. I've been using it for over a year, sparingly. I don't use large amounts of data when I tether, just email, surfing, etc...
HTH,
Brian