In Aperture I use "Vault" feature to make backup its will be good thing to add to PM, I miss backup feature, also way to make multiples backup volumes like "dailly" and "weekly" backup, in think can be use rsync as backend.
Apple has included the ability to backup an Aperture Library into a Vault. A Vault is an exact copy of an Aperture Library. Once a Vault is first created, any additions or changes to the Aperture Library are made to an associated Vault incrementally during the synchronization process. The program can backup a Library to one or more Vaults, with the incremental updating of multiple
I think Apple implemented their Vault backup utility because Aperture is a "do it all in one app" style program and it does not really consider working on your images with anything else. This is why they keep it all in their library and why they provide this backup.
PM is designed to work fine with other programs and except preferences it does not really store any data elsewhere but the image files you manage with it. So you can use any kind of backup sofware to protect your images even without PM itself providing such a feature.
However PM does offer a backup feature during ingest to be able to make a redundant copy of your images before they get erased from your cards.
On the Mac there is Apple's
Time Machine looking every hour for your changes and doing a backup.
I prefer
CrashPlan which seems me more powerfuland advanced to me and it is multi platform supporting OSX, Win and Linux as well. It can operate over the internet (not adviced for mobile Internet with low bandwith and large image files) and USB as well. And most of all it supports what you mentioned: once a file is changed it backs it up to the given backup location(s).
