You're not the only one having issues. What's weird is that I created a boot disk for Lion and had no issues with it, but I can't seem to create a USB disk that works for ML.
When I erase my USB disk it still shows a couple of partitions. Then when I try to restore InstallESD.dmb to the USB disk it never works. I have to eject the partitions showing for the USB drive and then it will try to restore, only to fail several minutes from completion with a "invalid argument" error.
I was able to restore the Mac OS X Install ESD (by mounting InstallESD.dmg and then explicitly doing restore on the Mac OS X Install ESD partition) to the USB drive, but this resulted in a drive that never seemed to boot ML fully. It would present itself as a bootable disk but it would just sit there during boot with the Apple logo eventually becoming a blank icon and I finally just hard rebooted the computer.
I will start scouring for other instructions on making a boot disk for ML because I'm not having much success with this approach.
Also as someone else pointed out, while someone taking the time to create a guide is much appreciate, it seems that there might be some steps that are missing or not particularly well articulated, not to mention the use of at least one "screenshot" which shows disks/partitions that have nothing to do with the task at hand, which leads to further confusion.