The Dev-Team has released another beta of the “back-up plan” 4.2.1 untethered jailbreak. This updated version eliminates the need for usbmux, which caused major problems for some users. As with the previous redsn0w betas, this is meant for experienced beta testers using an iPhone 4, iPod Touch 4G, and iPad only.
The Dev-Team plans to release a Windows version and open up the program to other iOS devices with the next major release.
The fourth beta is now available (testers ONLY please!). Starting with this version, redsn0w no longer requires you to install and run a separate program for the usbmux (this was a non-trivial procedure for many). It’s okay to keep that program running if you normally use it for ssh anyway. We also think we’ve fixed a race condition in the GUI that was causing redsn0w to crash during the “Preparing JB Monte Files” step. Nothing at all has changed on the iPhone side (where the main issues are still just Bluetooth and Skype).
Hit the source for the download link and full details.