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.

Well i got a panic call from the Inlaws that their Imac hangs on boot-up. It will hang and then a tiny folder appears in the middle of the screen with a flashing question mark/and that os face icon.