I've just learned about the 'Drobo'. This thing is incredibly cool. It's a big black box into which you slot in hard drives. If a drive fails you just pull out the old drive and put a new one in. You don't have to switch it off. You can mix hard drives, so as you need more capacity you pull out an old drive and put in a new one. In the demo, the chap plays video from the drobo, whilst pulling hard drives out of the box - all without a break in the video stream. As far as the computer is concerned, it looks just like a single drive. The box handles data redundancy, so you can pull out an old drive without losing any data. Running out of space? A light comes on next to an empty slot and you just pop in a drive (at current internal HD prices). Four drives? The light comes on next to the smallest drive (which is usually the oldest and hence most likely to fail) - change it, without even powering down and it just works.
This looks like a great tool.
Backup is important, and 'time machine' for the Mac is great - it's enough to make me think 'swap to mac', and coupled with drobo this has become 'swap to mac, get a drobo'.