This link is unwarranted and only for geeks folks, but if you want to see how it works, check out Brian Tong's video on CNET TV.
Here's the link : http://appldnld.apple.com.edgesuite.net/content.info.apple.com/iPhone/061-4955.20080710.bgt53/iPhone1,2_2.0_5A347_Restore.ipsw
Update: Many people are reporting problems with the unofficial iPhone firmware. If you're having issues or just want to avoid them, here's how to upgrade your iPhone to the official firmware update (via MacRumours).
1. Sync your iPhone with iTunes 7.7 to make sure all data is backed up.
2. Download iPhone1,1_2.0_5A347_Restore.ipsw (Official firmware). You should get a single ".ipsw" file. If your browser renames it to a .zip file, you should rename it to ".ipsw"
3. In iTunes click on the "Check for Updates" while holding the Option key on the Mac (Shift key on Windows)
4. Select the firmware you downloaded in step #2
5. Wait for your iPhone to update and restore.