There is nothing exciting or worth upgrading in iPhone OS Update 3.1.3 but if you still want to upgrade and keep the jailbreak it is now very much possible thanks to updated redsn0w 0.9.4 utility by iPhone Dev Team, do keep in mind if you are using an unlocked iPhone do not upgrade as no unlock is yet released for iPhone OS 3.1.3.

Just moments ago iPhone Dev Team released the redsn0w 0.9 public beta download for Windows and Mac, supporting the latest iPhone OS 3.1.2 the cross-platform jailbreaking and unlocking utility for iPhone and iPod Touch also supports customizations like changing of device boot logo, recovery logo, and allows users to switch on “verbose” booting mode.

The free standalone utility does not use iTunes and no custom IPSW's are involved in the process, the current beta supports all iPhones and iPod touches except the iPod touch 3G having Apple firmware 3.0 and 3.1.2, direct download links are at the end of this post.