When I push the power button, there is no activity at all. I tried resetting the power manager first.

Nothing.

I am pretty sure the AC adapter, battery, and DC board/cable are all functioning properly. Here's my reasoning. The battery was at 0 charge. I plugged in the AC adapter and it charged up the battery fully. That would mean that the adapter works, the DC board works, the DC cable works and is connected properly, and the battery holds a charge. Let me know if I should have any reason to think that any of these could still be the problem.

I tried your suggestion of bypassing the power switch with no result. This would leave only one option--a dead motherboard. Which, of course, would suck. I see that this site has them available for purchase, but I am not guaranteed that replacing it would solve the problem. It's a 4.5 yr old computer, so I guess I'm gonna have to cut my loss at some point and upgrade. Let me know if you have any more thoughts. Thanks.