AT&T has indicated that they've built the cost of replacement phones into the acquisition, but still worth keeping in mind if you're a T-Mobile customer up for an upgrade in the next year or so.
Update: An AT&T spokesperson has emailed us the following:
Will T-Mo 3G handsets work after the transaction? Yes, T-Mobile handsets will continue to work for both 2G and 3G service. Over time, we plan to upgrade T-Mobile's 3G customers on the company's AWS spectrum to our 850 and 1900 MHz spectrum. But this will be done through the normal handset up grade process - and it will be done gradually over several years after the merger closes.
So, as we said, although it won't be until at least two years from now, existing 3G T-Mobile handsets won't work anymore. You'll probably have upgraded by then anyway! But if not, at some point that G2 is going to be pretty useless. [AP]