For example, if I wanted to help out with trying edge(in this case Quantal), but I wanted to be able to revert to a stable-ish system if some update caused a problem -- how would I accomlish this task?
Configuration for your applications will be in folders whose names start with a dot, in your home folder. Backing those up may be advisable, especially if you want to revert to Precise, as config formats may change in newer packages, and things.
System-wide configuration is in /etc. Ditto.
You can back up your package selections with
EDIT: If you don't have somewhere to do backups to, or you want to revert your system to exactly the way it was, package-versions and all, you have the following options (copied from my comments on the question):
For the release upgrade, there's the --sandbox flag for
If you choose certain filesystems at install time, you may be able to produce copy-on-write snapshots, enabling you to roll your entire system back to precisely its previous state. Note, though, that the only fs I know to be able to do this is btrfs, which is very slow to run package-management on, because