Perhaps this is an obvious question, but will the Mountain Lion installer from the App Store work on any Mac? E.g. if I download it and put it on an SD card on my MacBook Pro can I use that SD card to install Mountain Lion on a family member's white MacBook? I know that the OS discs that the laptops came with were hardware-specific. I'm just trying to avoid downloading and burning the installer again.
Once you have downloaded it from the App Store, but before you run it, you can copy the Install Mountain Lion app into the Applications folder of any Mac you have an Admin account on (and have signed into the App store as you obviously) and run it from there. If the Mac is not recent enough to install Mountain Lion the installer will tell you, otherwise it will happily install Mountain Lion for you. That's how I updated all the Macs at my place.
Note that once run the installer will delete itself from the Applications folder.