The iPhone Dev-Team has announced RedSn0w 0.9.15b1 which brings numerous new features including the ability to restore to older firmware not being signed by Apple.
how to jailbreak Ios 6:
http://www.iclarified.com/25146/how-to-jailbreak-your-iphone-4-using-redsn0w-mac-60
Upgrade to Ios 6 without Updating the baseband:
http://www.iclarified.com/25142/how-to-upgrade-your-iphone-4-3gs-without-updating-the-baseband-mac
Download redsnow:
http://www.iclarified.com/16424/where-to-download-redsn0w-from
how to jailbreak Ios 6:
http://www.iclarified.com/25146/how-to-jailbreak-your-iphone-4-using-redsn0w-mac-60
Upgrade to Ios 6 without Updating the baseband:
http://www.iclarified.com/25142/how-to-upgrade-your-iphone-4-3gs-without-updating-the-baseband-mac
Download redsnow:
http://www.iclarified.com/16424/where-to-download-redsn0w-from