There's no guarantee the phone is locked at all. Cingular has exclusive distribution and service agreements with Apple, but that doesn't mean that the device itself is SIM locked. My first instinct would be that it isn't, because why should Apple bend to cell phone games? It won't be able to make these fast enough, period, and when they branch out to the rest of the world, it probably won't be locked.
Instead, they'll probably just make you sign up with Cingular as part of the checkout process (like when you buy phones at Best Buy or Amazon.com), and then you can cancel your individual contract for $200, thereby getting your own iPhone for T-Mobile or Orange or whatever GSM operator you choose. Cingular gets paid, Apple gets paid, you don't suffer any worse than usual with a cell phone. It's win-win-win.
Hope this may be true but seems like non-Cingular customers will have to pay a high premium for the iPhone.