Here's a story for anyone who's come across an unusual progress bar during boot and my best guess at what it really is. The moral of my story: backup frequently and try not to turn off your Mac while it is starting up.
Five days ago, I was prepping my late 2008 MacBook Pro for resale when I encountered the ever-spinning gear at boot. Ridiculously enough, I hadn't kept up any of my backups in a month and I hard powered off the machine while it was booting because I forgot to select the boot drive (yes -- I'm a technician and I did something dumb -- I know). Many thoughts went through my head: I've lost the last month's worth of work, I have to waste hours trying to recover data from my drive, why did this have to happen while I was trying to back it up? It goes to show that it can happen to anyone at any time and that the only real solution is to keep a current backup (if not several) of all your information.
Now for a quick word on backups. I usually keep a continuous Time Machine backup of everything and clone my internal drive every couple weeks. The cloned drive is kept in a fireproof safe here in my house. Also, as much as I love Time Machine, in my profession, I've seen several instances where Time Machine backups don't restore properly. I always suggest having a manual clone as a backup to your backup.