Cingular's got the largest customer base--it's a no-brainer. That said, they'll definitely offer deals with other companies. First, they'll have to; second, the on-screen display specifically lists which carrier you're using, which implies that multiple carriers are supported. If you don't want to wait, Cingular lets you unlock phones if you can prove that you paid full retail for it.
As a T-Mobile user, I can't wait to pick one of these up.
I have the Tmobile service too.
You don't need Cingular to unlock the GSM phones.
There are a number of services which will do it for you by provideing the code for less than $5.
Also there is software out there too.
There is no reason Cingular needs to be a gate keeper on your property.