Booting from a Time Machine drive
One of people biggest complaints with Apple's new Time Machine is that you can't boot from your backup. If the worst should happen and your system goes down, booting from your install DVD and restoring your entire system wastes precious time. What if you could use your computer while your restoring your main hard drive or partition? What if you would rather try to fix your broken installation instead of just restore it?
It turns out that there's no reason a time machine backup and a full bootable OS X installation can't reside on the same hard drive or partition.
You can either install OS X on the backup drive then use it for time machine, or use it for time machine then install OS X on it, so long as you have enough space to do so.