Click to viewThe iPhone 1.1.3 Jailbreak is here, courtesy of Nate True and the iPhone Dev team. It's a Windows-only download right now, but Nate tells us that they've got a Mac version coming soon and it "should be quick". Make sure you have more than 300MB free, otherwise Nate tells us that you might brick your phone during the update. Updated: Mac version's done and Lifehacker's got a walkthrough.
Fot the Windows version, you need to download an install iBrickr (instructions are on Nate's site), which will guide you through downloading and installing the firmware. The whole thing will take about 15-30 minutes, based on estimates.
Adam from Lifehacker has a walkthrough for the process as well.