When I was in high school, I remember I begged my parents for months to buy me a computer. They finally agreed, and I ordered a 386 SX 33Mhz computer, with a floppy and no CD drive, from a company called ExpoTech. That was the best computer I could afford. Well, they ended up sending me a 386 DX 25Mhz, with a floppy AND CD drive. I did the right thing, and called them. They promptly sent me the right computer, but no instructions to send the old one back. I called them again, and they assured me they would send a box to send it back. Long and short is they never did, and then a couple months later, they sent me a check for the full amount of the computer! So i had two computers and a check for 1100$.
My parents didn't let me keep the check, and I felt really good about myself at the time for doing the right thing.
All that being said - to the op: HELL YEAH I'D KEEP THE MONEY! SCREW AT&T! Now, for practical reasons, I'd say leave the money alone for a few months in that account, just in case they reverse the charge or come after you for it, but after that - you're free and clear.