Although AT&T and T-Mobile are both GSM carriers, they use different frequency bands for their 3G networks. This means that a T-Mobile device can't get 3G on AT&T's network, and an AT&T device can't get 3G on on T-Mobile's network either.
Using this information, it doesn't come as a surprise that AT&T has told T-Mobile customers that, if the merger becomes a reality, their devices will need to be replaced with AT&T 3G-compatible devices. But as a disclaimer, company CEO Ralph de la Vega did let them know that the transition won't happen overnight and that it should actually take a few years for it to conclude.
What transition? It seems that AT&T has decided to totally rearrange how T-Mobile's towers push out signals. So this kills any plans for a device with compatible bands for both networks, which is kind of a downer for anyone who would like a true world phone that they could also take advantage of at home.[Yahoo! News via GottaBeMobile]