Redsn0w 0.9.6 Beta 5 is out. Dev Team have just rolled out updated redsn0w to jailbreak iPhone 3GS 4.2.1 and unlock it with ultrasn0w 1.2. You can download redsn0w 0.9.6 B5 from Dev Team blog now. It comes with option to update the iPhone 3G and 3GS baseband to 06.15.00. So, you can update 05.14.02 & 05.15.04 to 06.15.00 with Redsn0w 0.9.6b5. Be sure to know the consequences though.
Here are links to download redsn0w 0.9.6 Beta 5:
Redsn0w 0.9.6b5 for: Windows [Mirror] | Mac OS X [Mirror]
alien says: 
