I have a 2007-era iMac in which I recently (~1 month ago) replaced the hard drive. It's been running just fine since. Today, I needed to reboot it in order to run Lion off an external firewire drive.

I realize from the outset this is going to be one of those unsolvable problems because it's so entirely random, but I thought I'd throw it out there in case someone has seen similar behavior. 2009 27" ...