Redsn0w 0.9.6b5 has been released for download for both Mac OS X and Windows, and it allows you to jailbreak iOS 4.2.1 on iPhone. This new version of redsn0w is particularly valuable to those wanting to unlock their iPhone from baseband 05.14 and 05.15 because it prepares the iPhone 3GS and iPhone 3G for unlocking with ultrasn0w.
Redsn0w might also be considered easier to use since the application will automatically download the iPad IPSW 3.2.2 file from Apple rather than you having to do it yourself, meaning you only have to point at one IPSW file, which is your own jailbroken version.
Download redsn0w 0.9.6b5
Be sure to keep your device plugged in and powered via USB throughout the entire jailbreak process with redsn0w.