Im a T-Mobile Customer and the T-mobile version is unlocked but comes with a t-mobile sim. This version also does not require you to verify and account. If say someone wants to buy a phone out-right on launch day and doesnt want to verify an ATT account they could technically buy the t-mobile phone and then just pop in the ATT sim.
I guess if us AT&T people would have purchased the T-Mobile one, then we could get around the activation fee. Never thought of that when I purchased the AT&T one at full price.