Here's where things get interesting. Verizon pretty much has to allow FaceTime over Cellular because they are licensed in the C-block of the 700 MHz spectrum which has open network requirements. All of AT&T's licenses are outside of the C-block. For now.
The FCC released a public notice a few days ago that AT&T was attempting to transfer licenses from a company named CenturyTel. 4 of theses licenses are in the C-block. If AT&T were to acquire these licenses, they would be required to abide by the open network rules (at least in those areas of the spectrum.) I find it difficult to believe that AT&T would spend the effort to selective permit/deny that application depending on which frequency your phone was on.
I filed a Petititon to Deny with the FCC basically saying that I request that they deny the transfer of license unless AT&T agrees it will abide by these rules. It's important, in a pleading to deny, that you directly tie your argument to the proceeding. For example, you can't argue that AT&T sucks so you should deny it. You must argue that the transfer of the licenses to AT&T will have a direct negative consequence (such as the C-block regulations being violated) and should not be permitted unless conditions are agreed to.
If you want to and you don't have an FRN (if you have a ham radio license, you have an FRN too), then you can mail your opposition. I'm not sure whether or not you have to have a license of some sort to apply for an FRN.
If you do have an FRN, you can (publicly or confidentially) file it via ULS and then you also need to email it to 5 email addresses listed in the public notice. I submitted mine as confidential because I had my name/address/phone number in it.
The public notice can be found here:
http://transition.fcc.gov/Daily_Releases/Daily_Business/2012/db0911/DA-12-1479A1.pdf