It's a problem that's at least two years away, but AT&T has confirmed that 3G T-Mobile handsets won't be able to access the network once T-Mobile's towers are repurposed to drive AT&T's 4G LTE push. 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]