UPDATED JULY 2011- you can also do the SAME with iOS 4.3.3 by flashing the baseband, all rules apply, except you will browse for 4.3.3 firmware instead for your device.
Download Redsn0w RC16 for 4.3.3 for Windows Click here
Download Redsn0w RC16 Mac 4.3.3 for Mac Click here
iPhone 3GS 4.3.3 Jailbreak Download:
Mirror download: