Has anyone upgraded to the ML GM with a clean install? If so, how'd you do it?
Depends where you got the image from. If you have it from Apple's seed release you have to right click and go to "Show Package Contents" and look for a file called InstallESD.dmg. If you got it from an unofficial download it may already be the InstallESD.dmg that you're looking at. Once you figure that out you can restore a USB using disk utility and picking the InstallESD.dmg image as the source.
If you want to burn a disk, I'm guessing it would work by burning the InstallESD.dmg using disk utility.
This image file is found in Contents>Shared Support. Not entirely positive on that though.
Then after you boot it up, use Disk Utility in the installer to delete the primary HD where your last OS existed and then exit out of DU and click "Reinstall OS X"