Bought your WP7 device from AT&T before the in-store BOGO offer? You still qualify
Microsoft has felt the pain of those early adopters, who got their WP7 handsets before it introduced a BOGO campaign jointly with AT&T. The official Windows Phone 7 Twitter account states that you can visit an AT&T store, and still take advantage of the offer, if you have purchased your phone in the last 30 days.
Considering that the mobile OS only launched November 8th in the US, that pretty much includes everybody who ordered a Samsung Focus, LG Quantum or HTC Surround since then. Or, alternatively, instead of haggling with the rep, you can just go to Amazon and get any of the WP7 trio on AT&T for $49 and change.