iMac Grey Screen of Death?!

Looks like no apples here. I booted my iMac this morning only to see a circle with a cross through the middle sign instead of the usual Apple logo. I could only compare it to the common Windows BSOD blue screen of death. I was stuck. I laughed figuring this was more of a photo-op. Luckily the OS install disk was available. After running a repair on the Mac partition with Disk Utility the computer was back up and running 15 minutes later.