Time Machine saved my life!
OK, maybe it didn't quite save my life, but it worked just in time. Time Machine is a new feature in Mac OSX Leopard. It automatically backs up your system every hour, and stores one day's worth of backups well as one daily backup for the past month, then one monthly beyond that.
I loaded Leopard into my Mac Mini in mid November, and set up Time Machine right away. At some point in December, my computer started acting funny. Turns out, my hard drive was kaput! (Thank goodness for AppleCare) After getting my newly fixed Mini, I went back to restore from Time Machine, and there it is... everything that was there before...
Now, it didn't go without a hitch - I was having problems starting my Mini before the breakdown because it was unable to find the system volume folder at starup. I had to go back to a day before the meltdown to find a backup that included the correct folder. It took a while to load each iteration of the backup, but ultimately, I didn't lose any data and all of my preferences are set as I had them.