From what I have read on other threads, AT&T has a way to figure out that the SIM is being used on an iPad and not an iPhone; they will probably give you a courtesy warning before they take some drastic steps.
I am in the same boat as you, unlimited grandfathered AT&T data plan but would much rather use it on the rMini; hopefully iOS7 jailbreak comes out soon and tethering is workable again