The Blackra1n jailbreak for iPhone OS 3.1.2 is now available thanks to geohot. It covers all iPhone and iPod Touch generations and is quite simple to install. Download the Blackra1n and install it in your PC and then plug in your iPhone or iPod Touch. Be careful though. If you have previous iPhone OS versions like 3.0.1 or 3.0, do not update. Your unlock feature will be lost. Use the Pwnage Tool to update to 3.1 first and then wait for the next version of Pwnage before updating to 3.1.2.

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(Source) Gizmodo