In order to fix minor issues arising out of Redsn0w while downgrading iOS 5.1.1 to iOS 5.1 or iOS 5.0.1, iPhone Dev team has released version 0.9.11b4 of the jailbreak tool that fixes the strange “Restore failed” message, some users were getting. Redsn0w now doesn’t interfere with nearby devices that have WiFi syncing enabled. In addition to it, Redsn0w 0.9.11b4 provides tethered jailbreak support for iOS 5.1.1 (build 9B206) on A4 devices and earlier, including proper “Stitching” and “Custom” creation of NO_BB IPSWs.
iPhone 4 and iPhone 3GS owners who preserve their baseband, the Redsn0w 0.9.11b4 can produce the NO_BB_* custom IPSWs for 5.1.1. They can use redsn0w’s “Custom IPSW” button to create a NO_BB_* version of the 5.1.1 IPSW and restore to that IPSW instead of the stock one. If you try restoring your iPhone to a stock Apple IPSW, your iPhone will be locked. This is still a tethered jailbreak for all except old-bootrom 3GS users.
We will soon be posting guide on how to create custom iOS 5.1.1 IPSW (NO_BB_* version) and restore the custom firmware on iPhone 4 or iPhone 3GS without upgrading its baseband. You can then jailbreak your device with redsn0w and unlock it