With the latest version of Redsn0w, developers don’t have to use the trick of pointing to iOS 5 firmware file.
iPhone Dev team have provided the following information on their blog:
redsn0w version 0.9.9b8 will natively recognize the 5.0.1 beta that Apple let developers start testing this week (so you don’t have to play the “point at 5.0 IPSW” trick). Because the IPSW isn’t public, you’ll still need to provide it once to redsn0w (at which point it will cache all the important pieces so you don’t need to select it again).
For those 3GS and i4 users who preserve their baseband, the Mac version can also produce the NO_BB_* custom IPSWs for 5.0.1. (But please note that ultrasn0w doesn’t get updated for betas like this.)
This is still a tethered jailbreak for all except old-bootrom 3GS users.
Redsn0w 0.9.9b8 supports the following iOS 5 devices:
Redsn0w 0.9.9b8 supports only a tethered jailbreak for iOS 5.0.1 just like iOS 5, except for iPhone 3GS with old bootrom which is untetherd. It doesn’t support iPad 2 and iPhone 4S. After developers have jailbroken their iOS device, they can use the SemiTether hack, which allows users to reboot their tethered jailbroken iOS 5 device without the need to connect it to the computer.
[via Dev Team's blog]