Source: http://feeds.gawker.com/~r/gizmodo/full/~3/313190879/iphone-3g-att-activation-details-doing-it-at-home-is-possible-but-requires-a-promise
The two things we previously heard about iPhone 3G activation on AT&T—mandatory in-store activation and a penalty if you don't activate within 30 days—seemed to contradict each other. Not anymore. Apple Insider has just heard through an internal memo that people can actually leave a retail store without activating their iPhone 3G as long as they've signed a form saying that they intend to register with a 2-year service plan within 30 days. The retail guys would then input the IMEI number of your phone into their machines, which would enter AT&T's Death Star tracking machine to allow them to know whether or not you've properly activated. Now it all makes sense. [Apple Insider - Thanks Joel!]
And here's our gigantic FAQ about the iPhone 3G.
