I'm not really so dumb as to leave off the heat sink, but I *was* dumb enough to leave the little plastic sheet over the thermal pad. Actually, I pulled it off as the instructions said, but it left a second layer underneath. Not realizing the pad itself was supposed to be pink instead of blue, and the instructions DID say a "colored pad.." well, it was the first HFS I had used that came with the bubblegum.. excuses excuses, right?
Anyway. Time-to-destruction for a 1.1GHz Thunderbird with single layer of plastic between core and thermal pad: literally, 10 seconds. It posted, I dropped into BIOS settings, and it locked up immediately and never posted ever again.
-Rob