It has alreay been talked upon that iPhone will be carrier specific. But now I feel it will be GSM only and may be later it will be for CDMA also. This is a step in the right direction, because by then the market reacion will be clear.
You are so right, as now that you've mentioned it, I do remember that the AT&T exclusive is for two years, with an option to renew.
Fortunately I'm already with AT&T so it's a non-issue for me, it's the others that will have to make a "bigger" decision if they are willing to pay the price to move from their current service provider to AT&T.
Furthermore its quite positive that AT&T was chosen since Apple decided on releasing it through just one carrier, as they have the biggest subscriber base and the most resources to promote the phone.