I'm willing to bet that it won't work. The reason:
Cingular has been bought and rebranded as AT&T in the US. So therefore Cingular and AT&T phones (except the iPhone) at pretty much the same thing in the US. This would be the equivalent of taking a Rogers phone and using it on FIDO (or vice versa). Not unlocking it for any carrier, but merely allowing all variations of the same service to access the phone.hacktheiphone.com said:
It's also noticeable because they don't say that they can get T-Mobile SIM cards working - if they could, it would be a full unlock.