If you get an iPhone, you can jailbreak it and download a cracked Cydia repository which will add some sort of hotspot app that has been cracked and is free. Pdanet is what I use, you can "mask" the usage from att.
If you need help with jail-breaking it shoot me a pm. I do it to phones a few times a week.
You can do it with pretty much all the new Android and Iphone's these days you just have to be rooted (and sometimes running a custom rom) if you're running android and Jailbroken on Iphone. Or AT&T is going to charge you and extra $20 dollars a month for tethering fees.