The first jailbreak for iPhone OS 4.0 has been released (Mac only), it is an updated version of Redsn0w but be warned that it is a test version only intended for jailbreak app developers. This jailbreak is for iPhone 3G and is not suited for end-users due to the fact that its very buggy.
Update: The full step-by-step guide to jailbreak iOS 4 on iPhone 3G can be found here. The guide below is for older iPhone OS 4 Beta 4.
Update 2: Redsn0w 0.9.5 has been released for Windows.
Note: Do not try this on the iPhone that requires carrier unlock. This app does not hactivate or unlock your iPhone, thus, do not upgrade to iPhone OS 4 Beta at this moment.
Note 2: Do not try this on the latest iOS 4 yet. It has not been proven to work on the latest GM release.
Note 3: If you have iOS 4, stay away from Unlocking it. You can only use iOS 4 with iTunes 8.2.
Final Note: MuscleNerd has tweeted that Redsnow 0.9.5 can jailbreak iPhone 3G running iOS 4, this is what Redsnow wiki has to say,
“For those who can legitimately activate to the iPhone3G 4.0GM seed, the “point to previous IPSW” redsn0w trick works and will jailbreak your 4.0GM. The “trick” is to point redsn0w at the 4.0beta4 IPSW instead of 4.0GM (do this after you’ve updated to 4.0GM and activated through the new iTunes 9.2 on your Mac). Since the carrier unlocks aren’t yet supported at 4.0, don’t do this if you use yellowsn0w/ultrasn0w/blacksn0w. Many JB apps are still pretty buggy in 4.0, so again this is mostly useful only for developers of those JB apps.”
For the full step-by-step guide, go here.
Here are some instructions from the Dev Team wiki:
- Make sure you’ve already activated your iPhone3G with iTunes and your developer UDID
- Launch the beta redsn0w 0.9.5
- Select your stock iPhone1,2_4.0_8A230m_Restore.ipsw file (you’ve already used this to update your iPhone3G to 4.0beta1)
- Select “Install Cydia” and click “Next”. Use DFU mode to install the jailbreak.
- When your iPhone3G comes back up, you’ll notice Cydia has a blank white icon (we told you many things were broken right now!) You’ll also notice Cydia has no sources. Go to the Sources panel and add this repo: http://apt.saurik.com/cydia-3.7 (be sure you have a carrier or wifi signal first!). When Cydia restarts, you should see its real logo now, and the standard sources should be ready to use.