William Wiley was a passenger on US Airways Flight 1549 that landed in the Hudson River last month. He had a laptop with him with about $30000 of data on it. He had a backup of this data on another laptop. However, he had this backup laptop with him on the flight. Both laptops are now in storage, awaiting data recovery which will take about eight weeks.
Backups are a great idea. But you don't want to have your only backup copy of your data stored with your primary data device. You want to keep at least one backup in a different location. I have two backups of the data on my Macbook Pro: a Time Machine disk at the office that backs up the computer every weekday, and a SuperDuper backup at home that holds a recent image of my laptop (at the moment this image is about a month old). I also back up my photos and music on my iPod. I wasn't always this conscientious about my backups. But I lost all the data on my PC a few times when the drive crashed, which cost me about two years' worth of old e-mail and old documents. Now I'm paranoid about backups because you never know what's going to happen.