Originally Posted by odysseus

It turns out that the AT&T sim card that I have does trigger a proper activation on my iPhone 2G, so that I would expect that after running redsn0w with that same sim card inserted, I should then be able to insert my T-Mobile sim and use my phone.

Try it out and report back. I am not 100% sure - I just know that Apple changed the activation system with the 3G.