I was, just like many others mislead into believing the phone has AT&T bands as well. Carriers, retailers and even OEM's are notorious for listing wrong specs.
From the horses mouth:
The third page of FCC's test report (download the PDF) doesn't list AT&T's 3G bands anywhere. I am positive that they would if the device has them.
Those of you on AT&T or other carriers should wait for official confirmation or until someone unlocks the device and tests it.
As for T-Mobile customers, the completion of the merger and shutdown of T-Mobile 3G bands is at least 18 months away.
The G2X is great phone however, roughly 18 months from now when AT&T hands out replacements, the device will be aged compared to the phones that will be sold at that point. You can look at the howard forums for feedback on what replacements Alltel customers were given when Alltel merged with AT&T.
My point is that T-Mobile customers shouldn't take AT&T compatibility into account when purchasing a device.
But, for those on Even More+ plans (no contract plans) keep in mind that you will not be given replacement phones. Your AWS phones will become useless. AT&T promises to offer deeply discounted phones for their prepaid options, though their prepaid plans are more expensive by default, data in particular that smartphones rely on.
Regardless if you are buying a phone outright or using EIP, the clock started ticking for off contract customers, I wouldn't buy a AWS smartphone past this summer, you are likely to get only a a year and half out of it if you buy it now.
Alltel's arguably best phone was the Desire. Here is a link to a thread of what options for free replacements Alltel customers were given if they wanted to stay on their plans:
Phone: HTC Leo (T-Mobile)
Peter Chou, HTC CEO: "No Android on HD2"