You can see geohot's message in the attached picture (click on it to enlarge). So jailbreak for 3.1.3 firmware it is not as difficult as some people want to believe and the only benefit is for Apple Lossless Audio CODEC (ALAC),.
However, geohot has a rather harsh attitude towards those who have/will update to 3.1.3 firmware, but the owners of iPhone 3GS they have no choice but to update if they want to restore because Apple Lossless Audio CODEC (ALAC), blocked the signing process a firmware 3.1.2.
Although this message mostly shows that we will not see a jailbreak solution from Geohot, I still say that we will see a blackra1n for 3.1.3 firmware soon enough. Until then, keep close, the tutorial for use will be ready soon sn0wbreeze cu 3.1.3 firmware.
