Update: it is over
Today, Apple began allowing downgrades to iOS 6.0.x which is quite unusual and will probably be changed very soon. If you upgraded past 6.1.2 and can’t jailbreak now is your time to downgrade and jailbreak before it is too late.
1. Download the 6.0.1 or 6.0.2 firmware for your device.
2. In iTunes hold the (alt key on Mac / shift key on Windows) while clicking restore.
3. select your firmware and downgrade.