Dev-Team released RedSn0w 0.9.9b3 to Jailbreak the iOS 5 GM firmware. The new improved version of RedSnow features iOS 5 GM firmware detectionto avoid having to point RedSn0wto an earlier iOS 5 beta like in version 0.9.9b2.

You can follow the steps on how to jailbreak iOS 5 on iPhone 3GS and iPhone 4 at

You can download the RedSn0w 0.9.9b3 for Mac OS X at Download RedSn0w (14.57 MB).

Source: ijailbreak