As a recent Mac convert, who plans to do Mac software development but hasn't started yet - is there a way to copy the existing mountain lion install on my Mac to a VM image so I can test on ML after I upgrade to Mavericks?
You can: I upgraded my Mountain Lion test machine to Mavericks when DP1 landed and created a Parallels virtual machine for Mountain Lion.
However, I found—at least with Parallels—you can't just take an existing hard drive image and use that: you have to create a new virtual machine, install Mountain Lion from scratch on it, then use OS X's Migration Assistant to move everything over.
You'll need to re-download it from the App Store (it should automatically be flagged as purchased, though, so you shouldn't need to re-buy it: just check the "Purchases" tab).
It'll download as an application: "Install Mac OS X Mountain Lion.app". If the installer automatically launches, just quit out of it. Right-click on the application, select "Show package contents", then navigate to Contents/SharedSupport. In there, you'll see "InstalESD.dmg": that's the Mountain Lion image.
You should be able to use that image directly in Parallels et al, but if you really wanted physical media, you can burn that image to a disc or create a bootable flash drive using Disk Utility. If you go the flash drive route, you'll need to use the GUID partition table.
I successfully installed Mac OS X in VirtualBox using iAtkos ML2, then upgraded it to the latest Mountain Lion through the App Store. You could probably do this, then arrange to mount your current drive and use Migration Assistant.
I might be doing this myself. Google "iAtkos ML2 VirtualBox" to find the settings for VirtualBox that will work.
