Few days ago, the iPhone Dev-Team has released Redsn0w 0.9.9b9 and Ultrasn0w 1.2.5, both comes with major upgrades with full support to the latest iOS 5.0.1 firmware. However, there was a bug in the Redsn0w 0.9.9b9 version reported to the Dev-Team by number of iPhone 3GS owners with old bootrom.
Fortunately, the issue related with the iPhone 3GS untethered jailbreak on 5.0.1 firmware has been noticed, the iPhone Dev-Team has just pushed out the fix in an update with Redsn0w 0.9.9b9d for Windows and Mac both. Notably, only iPhone 3GS users with old bootrom can perform untethered jailbreak on iOS 5.0.1 firmware using Redsn0w jailbreak tool.
Download Redsn0w 0.9.9b9d for Windows
Download Redsn0w 0.9.9b9d for Mac OS X
via Xiphone