This discussion of backup strategy has overlooked an important point, namely that Rita is considering upgrading to Mt. Lion with a view towards possibly reverting to Lion, and if she does decide to revert she'll need a Lion recovery partition (in lieu of an install disc), so how does she perpetuate hers?
And more important, will she even be able to restore Lion after having upgraded, or will the App Store have pegged her Mac pegged at Mt. Lion, costing her the ability to revert via that avenue?
Time Machine can restore to a prior OS. Let TM do a full disk restore to a time when you were on the prior OS, then let TM recover your home folder (or perhaps just your documents folder) back to its latest state. Let the App Store bring your apps back up to date. But frankly, the difference between Lion and MtLion is so slight that it's unlikely you'll ever need to roll back. Especially if you're a new user and don't have a lot of cruft anchoring you in the past.
The App Store still offers to let me download Lion (because I've purchased it) even though I've moved up to MtLion (which it also offers to let me download). In each case, what App Store sees is that I purchased the installer but no longer have it. It doesn't care that they're OS installers; to the App Store, they're just apps. It doesn't even see them as different versions of the same app.
