Why won't my unlocked iPhone 3GS connect to t-mobile?

I have an AT&T iPhone 3GS. I jail broke it and unlocked it through ultrasn0w on cydia. But it still won't connect to the network when I put in my SIM card. I also tried SAM, but nothing has worked. Can you help?

Why don't you get it factory-unlocked, check online for that. This guy (arghyle@rocketmail.com) claims to unlock iPhones and that might solve your problem.