iOS device users can actually jailbreak iPhone 3GS, 3G iOS 4 after the release of PwnageTool 4.0, with the stable and most trusted tool from iPhone Dev Team. Fortunately there is also no need to wait in involved just to unlock iOS 4 as ultrasn0w 0.93 does for you. These tools creates custom firmware which does not upgrade the baseband you have.

If you are interested for these exploring this custom firmware [ipsw] and you just want to restore it right away for jailbreaking, a created custom iOS 4 firmware [ipsw] for your iPhone 3GS and 3G is here for you, using PwnageTool 4.0/4.01.

These custom firmware are really different for unlockers and official carrier users. Choose your software here according to your requirements, also in what you need and follow this PwnageTool 4.0 instructions from iPhone Dev Team.

Download Custom iOS 4 Firmware for iPhone 3GS

3GS Custom iOS 4.0 Firmware [Unofficial Carrier IPSW (For Unlockers)] [iBooks Fix]

3GS Custom iOS 4.0 Firmware [Official Carrier IPSW] (iBooks Fix)

Download Custom iOS 4 Firmware for iPhone 3G

3G Custom iOS 4.0 Firmware [Unofficial Carrier IPSW](iBooks Fix)

3G Custom iOS 4.0 Firmware [Official Carrier IPSW] (iBooks Fix)