If your iPhone 4 is locked to AT&T or another carrier, good news -- the ultrasn0w unlock is now available to set you free.
Those who ponied up full price for an international, factory unlocked iPhone 4 direct from Apple need not apply. Those who chose subsidies and the lock that came with it, get your Jailbreak on and head over to Cydia. If ultrasn0w doesn't show up immediately, search for repo repo666.ultrasn0w.com.
Version 1.0-1 of ultrasn0w works for:
Remember, you'll need a microSIM card if you plan on jumping to another carrier, and in the US that means manually cutting down your T-Mobile SIM to microSIM size.
As always, Jailbreak and unlock will void your warranty and could cause other problems with your device. If you don't know what you're doing, don't do it. If you decide to go ahead, let us know how it works for you.