Last week I detailed how to jailbreak your iPhone to the 1.1.3 firmware using Windows- or Mac-based applications and walking through a fairly long process. If that process was a bit too daunting for you, the Jailbreak Dev Team has released a one-click solution that you simply install through Installer.app. To use it, you need to have a jailbroken 1.1.2 iPhone (here's how to do that), then it's simply a matter of tapping the install package and waiting about 20 minutes for the entire process to complete. Before installing, make sure you've set your Auto-Lock feature to Never, are connected to a Wi-Fi network (it downloads a 200MB image), and you've got at least 300MB of free space on your iPhone. Finally, check to make sure you've got the latest version of Installer.app before proceeding.
I haven't tried this method yet, but it should be scads easier than the previously posted method. As always, remember that things can go wrong, so proceed with caution. If anything does go wrong, check the panic section of this page to restore your phone to the 1.1.1 firmware and try again. As far as I know, this method will still require you to manually enable the Faux-GPS feature. Also, rumour has it that the current jailbreak method may mess with the upcoming third-party apps (though one would assume you could always downgrade and start fresh if you needed to), so weigh your options before proceeding. If you give it a go, let's hear how it worked out for you in the comments.