you cant if you have the latest baseband...
but may i suggest you take note of this and in the future buy an unlocked iphone or one thats locked to a carrier you plan to use, because then you wont be stuck relying on unlock tools just to save a few bucks..
A few bucks....
You can get a locked phone here for $200, while an unlocked one is around $800. $600 dollars is not a few bucks to most people I know.
I got my iPhone 4 as a gift at the beginning of October factory loaded with 4.1 and I'm on T-mobile (grandfathered unlimited calls, unlimted text, and unlimited internet for $40 so I'm not switching to ATT). Hopefully 2.10.04 will be unlocked soon. My poor iPhone has been living in the box for far too long.