For those looking for a simpler method to upgrade from a Jailbroken phone to 1.1.3, iJailBreak.com released iJailBreakMobile 1.0, which is an Installer.app application that allows users to upgrade from a Jailbroken 1.1.1 or 1.1.2 iPhone to a Jailbroken 1.1.3 iPhone.
To jailbreak, go into Installer.app, tap sources at the bottom right, then Edit at the top right, then Add at the top left, and add ijailbreak.com/repo.plist. Then, under the iJailBreakMobile category, choose the right upgrade package for you - if you're on a jailbroken 1.1.1, choose 1.1.1; if you're on a jailbroken 1.1.2, choose 1.1.2. The installation should take about 45 minutes, depending on your Internet connection. Once done, the device will try to reboot automatically. If you see that it's stuck with just the "spinning cog" on the screen, it's safe to manually reboot the device, at which point you'll enter into 1.1.3.
Jailbreaking is the process which allows you to install unofficial 3rd party applications on the iPhone. We have not tested this process yet, so proceed with caution.