An updated version of Redsn0w has been released today which brings support for updated .ipsw file for iPad 1G iOS 5 Beta 6

Update #7: Apple updated the iPad1 iOS5b6 IPSW without changing its version number or filename, so we’re releasing redsn0w 0.9.8b7 to handle both the original and changed IPSW. We’ve also added explicit support for a tethered 4.3.5/4.2.10 jailbreak (instead of pointing at the 4.3.4/4.2.9 IPSWs) and fixed a 4.2.10 problem.

Download redsn0w 0.9.8b7 For Mac
Download redsn0w 0.9.8b7 For Windows