Run RedSn0w and point it at the 4.0 IPSW. (Yes – point it at the 4.0 IPSW even though you're at 4.0.2). This will also hacktivate your iPhone3G at 4.0.1 if you haven't already been activated by iTunes.
If you need a carrier unlock - install ultrasn0w in Cydia (add repository http://repo666.ultrasn0w.com). For T-Mobile in the USA remember to turn off 3G mode in Settings→General→Network.
Run RedSn0w and point it at the 4.0 IPSW. (Yes – point it at the 4.0 IPSW even though you're at 4.0.1). This will also hacktivate your iPhone3G at 4.0.1 if you haven't already been activated by iTunes.
Install ultrasn0w in Cydia (add repository http://repo666.ultrasn0w.com) if you need a carrier unlock (and in the USA, remember to turn off 3G mode in Settings→General→Network).
DevTeam updated iOS 4 jailbreak utility redsn0w to version 0.9.5b5-5. That should fix any APN or MMS issues that users were seeing. It’s safe to re-run it on an already jailbroken iPhone without restoring. Just deselect "Install Cydia" if you do that.
Supported devices are still only iPhone3G and iPod Touch 2G (not MC)
DevTeam successfully jailbreaked new iOS 4. They updated jailbreak utility redsn0w to version 0.9.5b5-3. Supported devices are iPhone3G and iPod Touch 2G (not MC). New redsn0w is also able to hacktivate iOS 4.0 for iPhone3G.
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.
Note: If you're experiencing MobileSafari crashes, double check that you're running the version at the bottom of this page.
For those without legit iPhone3G 4.0GM seed access, please wait until the full public 4.0 release for updates to the jailbreak tools.
Last week Apple has released its iPhone Os 4.0 beta 4 and redsn0w is finally updated to support this as well as beta 1. New version of jailbreak is 0.9.5.b4 and it can be applied on iPhone 3G only(!).
Redsn0w 0.9.5.b4 is targeted at developers who make jailbroken apps, and will not work if you are already using blacksn0w, ultrasn0w or yellowsn0w.
iPhone Dev-Team warns:
"You should stay clear of this beta software if you rely on a carrier unlock."
To use the jailbreak, follow the guide below:
Download redsn0w 0.9.5.b4 here. (Only Mac OS X version since app are developed on this OS).
Make sure you have already activated your iPhone 3G with iTunes and your own developer ID.
Launch the jailbreak.
Select your stock iPhone1,2_4.0_8A274b_Restore.ipsw (beta4) or iPhone1,2_4.0_8A230m_Restore.ipsw (beta1) file that you used to update your firmware.
Choose "Install Cydia" and then click "Next". Use DFU mode to install the redsn0w.
When your iPhone 3G comes back up, you will notice Cydia has a blank white icon. It also has no sources so you should go to the Sources panel and add this repo: http://apt.saurik.com/cydia-3.7 (make sure you are connected to the Internet before). When Cydia restarts, you should see its real logo now, and the standard sources should be ready to use.
MuscleNerd just released redsn0w 0.9.5 beta - the first public jailbreak of iPhone OS 4. This utility is only for developers of jailbroken apps, so they can update apps for firmware 4.0. The jailbreak only supports the iPhone 3G and runs on MAC OS X. There is no carrier unlock right now.
Sn0wBreeze 2.9.3(pwnagetool for windows, supports tethered jailbreak of iOS 5.1 for iPhone 4, iPhone 3GS, iPod Touch 3G, iPod Touch 4G, iPad; untethered for iPhone 3GS with old bootrom; untethered for iOS 5.0.1, iOS 4, iOS 3 supports Apple TV 2G)
Sn0wBreeze 2.8b11(pwnagetool for windows, supports iOS 5, iOS 5.0.1, tethered jailbreak for iPhone 4, iPhone 3GS, iPod Touch 3G, iPod Touch 4G, iPad, Apple TV 2G; untethered for iPhone 3GS with old bootrom)
Current version is ultrasn0w 1.2.3, download in Cydia (unlock for iPhone 4, iPhone 3GS, iPhone 3G, firmwares 3.0-4.3.3; it supports basebands 01.59.00, 04.26.08, 05.11.07, 05.12.01, 05.13.04, 06.15.00)