I'm curious... How would you update to Mountain Lion without downloading it via the MAS..? And to have MAS, you'll need 10.6.8...? Your Restore DVD usually comes with a version of SL before that...
Or if you already have a bootable Mountain Lion to start with, why bother with a fresh install of SL at all...?
To satisfy your curiosity...
This is a 2nd partition. The first partition has 10.6.8.
I'm setting up a 2nd partition to play around with 10.8.
So I downloaded it via the App Store from my SL partition.
Had no memory stick or Dual Layer DVD to make bootable media from.
So I'm doing the install/upgrade via the SL restore disk, and then the ML update that I downloaded.