iPhone unlock

[Mac Intosh]

I am planning on getting an iPhone. I currently live in the U.S. and use T-Mobile. I want to buy an iPhone and unlock it to use T-Mobile on the iPhone without paying for AT&T. Is there a video tutorial on how to software unlock the iPhone? I tried looking but couldn't find a good one. Thanks.

EDIT: I know I would have to downgrade to 1.1.1 from 1.1.2 since 1.1.2 just came out today... so start from there. Also would it be possible to downgrade like restore the iPhone and load on a different firmware?