I'm not too sure but I've read somewhere that tmobile allows for tethering on their unlimited data plan while your required to get a tethering plan with ATT ( even though you could get away with it as long as you don't exceed a 5gb cap)
Posted from my CrackBerry at wapforums.crackberry.com
yes best thing about t-mo is everything is cover under unlimited data which includes tethering. but some say dont go over 10 GB/month...if u do they will slow down ur tethering speed down to like 56k. i dont know if there is any true to that statement about slowing down ur tethering speed. if that is even possible?