google's engineers are presenting a study of disk drive failure at a conference and the 13-page paper is online. If you don't want to read the whole report, TG Daily has a fairly good summary.
The most interesting observation is:
temperature and high usage alone are not responsible for failures by default. Also, the researcher pointed towards a trend they call "infant mortality phase" - a time frame early in a hard drive's life that shows increased probabilities of failure under certain circumstances.