During this evening, MuscleNerd aabridal through his official account on Twitter that there will be a new version of Pwnagetool compatible with 3.1.3 firmware soon, Dev Team working hard to perfect it and release it as soon as possible.
However, in that ad musclenerd warns users of iPhone 3G / 3GS to stay at 3.1.2 firmware if they don't need it urgently 3.1.3 firmware, this even after the launch Pwnagetool for 3.1.3 firmware.
This is a bit of a strange announcement considering that Geohot told ca jailbreak for 3.1.3 firmware it is not difficult at all and some just want to leave this impression on the users, but why? No one knows for now.
Of course it is Pwnagetool it will come soon and when it is released, you will be able to download it here and then I will also post a tutorial for creating a custom ipsw.
