If I were more demanding I would assume that, by now, a Mac would automatically copy the files from the .dmg over to the Applications directory when the user ru

If I were more demanding I would assume that, by now, a Mac would automatically copy the files from the .dmg over to the Applications directory when the user runs the program bundle for the first time. The programs would automatically get purged out after a timeout if the user never ran them again from the hard disk. Or even better, something akin to Zero Install (http://0install.net/ http://0install.net/) coated with OS X system sugar would be even better. That would also resolve the problem of software updates which are a pain on OS X, unless the program goes handle them by itself.