Steve Jobs has longed loathed the carriers control of the cellphone features, capabilities allowed, and music and video downloads, and would prefer, longterm, that the iPhone become "unlocked" and therefore carrier-free. The iPhone will have to eventually be unlocked for Europe and Asia, anyway.
But the iPhone could not work without a carrier deal. Remember how the US and European carriers refused to deal with Apple? Apple had to get this paradigm-shifting device into people's hands and therefore had to have a deal with a carrier. Remember T-Mobile's and Verizon's refusal to negotiate with Apple. The AT&T therefore required an "exclusive"...and that is no surprise.
But...if a third party, or even Congress "unlocks" the iPhone...then that's not Apple's fault for violating the exclusive contract with AT&T, and Apple would not need AT&T exclusively any more. Notice the "iPhone" hearings in Congress and the "Free the iPhone" movement in Congress?
Apple is playing a deep game here.
Why Apple Does Not Want the iPhone Unlocked and What It Means for Us