Yes if you are successful in jail breaking your phone you can get an app that will allow your phone to be a hotspot device using the plan you already have.
I think it would be great to use my laptop and iPad anywhere with the data I have now. I already have wifi at home and at work so this is mainly a mobile solution I wanted. I am hoping they come out with an untethered jailbreak for iOS7.I have 6.1.3 and it's a tethered solution.Tethered is bad if you need to reboot your phone away from a computer.
AT&T won't know. If you run into a problem you simply restore the phone from iTunes.Even if they knew it isn't against the law.