I've got multiple macs to upgrade to ML and I'd rather not download the massive update to each one individually. Is doing an in-place upgrade to ML as simple as extracting and copying the InstallESD.dmg to the target computer and running it from the desktop?
The easiest way is to copy the entire bundle "Install OS X Mountain Lion" to each of your macs, and run it on each of them.
Apple tends to be very good about this kind of thing. You can freely send it to all your macs legally if they're owned or controlled by you. You "may" have to sign into the app store on them before it'll let you install Mountain Lion, not sure, but it never gave me a problem when I sent my purchased iPhoto to my other mac that way. It should work fine.
I was worried about ML being tied to a specific Apple ID (like iOS apps) - especially since software updates act as if they're coming through the MAS now - but since it's not riddled with DRM like iOS apps I'm hoping this isn't going to be a problem. I'll stick the whole install app onto an external drive like you suggested, and see what happens. Cheers.
Nope. No activation, no DRM, no nothing. As long as you have the installer, you have it. Kind of interesting that Apple, that in so many other aspects are really "locked in", have this very laid back attitude to the risk of having their OS copied and illegally distributed...