iPhone Dev-Team has finally released PwnageTool 4.2 for an untethered jailbreak of iOS 4.2.1 on almost all iDevices except iPhone 2G. PwnageTool 4.2 is incorporated with the new kernel based exploit used in Chronic-Dev Team's GreenPois0n tool. It also comes with a fix for iBooks and Wifi related issues which will be available to GreenPois0n jailbreakers through Cydia package updates.
List of iDevices Supported by PwnageTool 4.2:
Although the actual injection method used in PwnageTool 4.2 still uses GeoHot's Limera1n exploit but the actual credit goes to 0naj, posixninja, and pod2g who worked on the second exploit involving Mach-o headers which brought untethered jailbreak to all iOS 4.2.1 iDevices.
For those who don't know, with PwnageTool 4.2, you will be able to create iOS 4.2.1 custom IPSW files for your iPhone which on restore through iTunes will jailbreak it on iOS 4.2.1 while preserving its baseband. Once your iPhone is jailbroken and its baseband has been preserved, you can use Ultrasn0w tool to unlock your iPhone.
If you have already jailbroken your iPhone, iPod Touch, iPad or Apple TV with GreenPois0n tool, you do not need to use PwnageTool 4.2 for an untethered jailbreak as the fix for iBooks and Wifi related issues is on its way through Cydia package updates.
The jailbreak instructions to use PwnageTool 4.2 for iOS 4.2.1 untethered jailbreak are similar to iOS 4.1 jailbreak through PwnageTool 4.1. Just follow the step-by-step guide here and use iOS 4.2.1 stock IPSW where needed.
Download iOS 4.2.1 for iPhone 4/3GS/3G, iPod Touch 4G/3G/2G & iPad
Download iOS 4.2.1 for Apple TV 2nd Gen
Download PwnageTool 4.2 for Mac OS X | Mirror
Download iTunes 10.1.2 for Windows and Mac OS X [via DEV-TEAM BLOG]