I suspect ATT doesn't know what phone your actually using, and won't try to find out if you buy something off contract -- then again since the Atrix is a '4G' phone, its possible the dumbphone data plan might not work for it or trigger a flag in their system. My Dad upgraded his phone once through a Walmart, and it was a year or more before our account showed the new phone model, though that was a feature phone.
If you ask ATT, they definitely will force you to change data plans. If you don't, my guess is that you'll stay in limbo a while longer at the mercy of ATT's sporadic policy enforcement/changes.
As for the data plan requirements, I also have the ATT Tilt (original), and it did require the $30/mo unlimited plan when I purchased it on contract. At the time though, the data plan was only required for a few months until your rebate cleared, and not as part of the contract.