UnCapped Internet

I'm an old att gophone customer with the last great data package att offered that i was grandfathered into....$20 unlimited interenet with NO data cap. i've been using any and every phone from blackberry's and htc's to samsung galaxy's. i've had phones imported from overseas to work on att's 3G band with NO problems. i've bee hesitant to get ANY iphone for fear they would detect me having an iphone and delete my data plan since you cant have an iphone on gophone and definitely not with that data plan. well i broke down have bought an iphone 4 off craigslist and want to get up and running. its att branded, locked, and has never been jailbroken. the Modern firmware is 04.11.08 and Version 5.0.1 (9A405). my question is will i be able to use this phone with my data plan as i've been with EVERY other phone??? Or will they detect me having an iphone and tar and feather me? i value the data plan more than the phone. any suggestions???