iPhone Dev Team has managed to release another update of their Redsn0w tool to version 0.9.9b4. This version is very powerful to jailbreak iOS devices. The download link is listed below. Here is the update of this version from Dev Team:
Update #12: For those of you who experiment with your own custom ramdisks using the -r command-line option, version 0.9.9b4 adds auto-detection support for iOS5. This is needed because iOS4 and iOS5 treat the root partition differently (it’s encrypted in iOS5). redsn0w will now upload the correct iOS5 kernel by itself, but it’s up to your own launchd to determine if it needs to mount using the old or new partition scheme.
redsn0w now also accepts both native and img3-encapsulated versions of files you provide via the -r, -k, and -d command-line options (do redsn0w -h to show all the available options).
Download link for Redsn0w 0.9.9b4: