While an official iPhone 4 Jailbreak just released a few days ago, now the Ultrasn0w carrier unlock was unveilved by the iPhone Dev Team, allowing US owned iPhone 4 users to use a different carrier than AT&T.
Update (October 10, 2010): New Limera1n iOS 4.1 Jailbreak Released by GeoHot for iPhone 4, 3gs, iPod Touch and also iPad
1. As always, Jailbreak and Unlocking your iPhone device is at your own discretion.
2. The new Ultrasnow iPhone 4 Unlock works on iPhone 4 on iOS 4.0.1 / 4.0 on 01.59.00 baseband.
3. For previous iPhone models, check out How To Jailbreak iOS 4 using PwnageTool 4.01 hacktivation: iPhone 3GS, iPhone 3G and iPod Touch 2G.
How To How to Unlock the iPhone 4 with Ultrasn0w
2. Now that your iPhone 4 is Jailbroken, you will have Cydia installed on it. So go ahead and start it.
3. Select “Manage” from the bottom of the Cydia home screen.
4. Select “Sources”.
5. Go to “Edit” and then select “Add”. In the URL box (screenshot below) enter “http://repo666.ultrasn0w.com” and confirm by “Add Source”.
6. Cydia will go through required source updates.
7. Search for “ultrasn0w 1.0-1” in Cydia and install the Ultrasn0w Carrier Unlock App.
8. Restart your iPhone 4.
Congratulations, you now have an unlocked and jailbroken iPhone 4.