Finally after much waiting, the famous iPhone hackers, Musclenerd and planetbeing (from the iPhone 4 Dev team) who have constantly been working on the iPhone 4 jailbreak/unlock situation have finally done it!
US iPhone 4 models that are locked to At & T Carrier, have now been unlocked i.e they can be used with any carrier you want. However the unlock and the Spirit jailbreak have NOT been released yet. They will most probably be released after Apple comes out with the firmware update for iOS4, to ensure the jailbreak and unlock still works after the firmware update.
Here are some pictures of the unlocked Apple iPhone 4: