If you're running Leopard and you've got an external hard drive, you will back up your data, even if that wasn't your intention when you bought the drive or plugged it into your Mac. That's because Leopard's system snapshot mechanism, Time Machine, is dead simple to enable—and Leopard prompts you to turn it on the moment you plug in your FireWire drive. You'll think, "why not?" and why not indeed. In System Preferences, Time Machine has exactly two settings: the On/Off switch, and what backup drive it should use. Once it's on, it creates hourly, daily, and weekly snapshots of your Mac, and automatically deletes the oldest ones when you run out of space on your drive—no user configuration required (or even possible, as far as I can see).
That terrible day you delete a file you need, to restore a past snapshot you'll use Time Machine's overwrought interface to fly back through space and time to that happy day when your file did exist. Gimmicky? Sure. But Mac guy John Gruber says that's a good thing:
Apple has made something so effect-laden and so extraordinary that users want to see it in action—the fact that that something is backups, which, let's face it, is effectively a chore, is a noteworthy achievement. Making backup software that people can't wait to try, and which, once activated, just automatically kicks in and does its thing on a regular schedule, is like making people want to go ahead and sign up for life insurance.
In short, Apple's used the best productivity trick in the world: to make the right thing to do the easy thing to do. Leopard's release will no doubt bring on an uptick of Mac users who diligently back up their system and data without even thinking about it.