The Dev Team just released Redsn0w 0.9.7b4, which eliminates the need to install and run a separate program for usbmux. But unfortunately this fourth beta is open only to testers only.
iPhone Dev Team has updated their blog post to provide more details about the new beta version:
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).
As it was long thought to cause redsn0w to crash during the “Preparing JB Monte Files” step. Bear in mind that this beta is not meant for public consumption just yet, so stay clear until all the bugs are ironed out.