As I understand it, Lion installs a separate small boot partition which can be used to run a clean install, disk utility, etc.
This is probably at its most important, not to mention convenient, when your HDD crashes and no data on it or any of its partitions are there anymore.
Good point. Personally, I use Time Machine backing up to an external Seagate. They're on a decent surge suppressor, but should both drives go at once I guess the OS disks will have to be dusted off. 🙂