I've blogged before about my problem with my PowerBook's Toshiba hard drive. It started the click-of-death and died without giving me a chance to back it up. I ended up managing to recover the data before. For more info, see my earlier blog entry: http://www.photoethnography.com/blog/archives/2005/02/equipment_drive_2.html
Since that incident, I've become more aware of the need to have a live backup. I'm so dependent on my laptop that a week without a computer just doesn't work. All of my lectures are on the laptop, my publications, etc. etc. I simply can't work without it.
I've now been taken to using SuperDuper! to clone my laptop drive occasionally to an external drive. It takes about 30 minutes, but it yields a bootable backup drive. I first used the program when my PowerBook started freezing up and I had to send it in (bad lower memory slot). I cloned the drive and then realized that having a bootable clone was a Very Good Thing. So I've kept at this, cloning/backing up every week or so.
Now cloning won't help you restore data that you accidentally deleted a month or two ago (unlike an incremental backup), but that's rarely a problem for me.