I was cleaning the Home Theater PC as I was moving and I took the processor out of the socket. It was stuck to the heat-sink. Naturally, I didn't want to disturb the great seal between the processor and the heatsink, so I just cleaned everything, put it all back together, and--before putting the heat sink/processor back into the socket--noticed that I bent one of the pins on the processor a little so I had to take an x-Acto to fix it. Whew!!!
Plugged it in and...
no POST, and not even a beep warning me about anything. F*around for a few hours cleaning everything extensively, removing all unnecessary components in case anything was causing issues, downloaded the manual to see if I missed anything, and then realized when I saw the first pages of the motherboard manual that:
I NEEDED TO PUT THE PROCESSOR INTO THE ZERO-INSERTION FORCE SOCKET AND THEN LATCH THAT INSTEAD OF JUST PLUGGING IT IN!
I thus pried off the processor from the heat-sink and installed it. Fired right up...
Lesson learned. Never assume anything.