no boot unless PS_ON line manually grounded

My D975XBX / Core 2 Duo E6600 system was running fine when I shut it down Friday night, then Saturday morning it was dead. The standby LED on the mobo was lit, but pushing the power button did nothing. The first thing I checked was the power button, since my case has been in service since the Pentium 3 days. The power button was fine. So I pulled the main ATX power connector from the mobo, grounded the PS_ON line, and checked all the voltages. They're all fine. So I assume that the mobo is bad. But then I got the crazy idea to ground the PS_ON line permanently with a paperclip stuck in the back end of the connector, just to see if I could get any kind of error beeps or anything out of the system, since it had been running fine until I shut it down and I couldn't find any bad caps or anything. I thought that maybe if I could get it to power up, I'd find I had a shorted video card or shorted RAM or something, and if not, what have I got to lose since I'm facing a mobo replacement anyway?

When I plugged the connector back in and turned on the standby power switch, the fans came on -- including the CPU fan, the drives spun up, i.e. everything happened normally except the system didn't boot. Then when I pushed the power button, it booted normally and runs fine! So basically, I'm wondering what I'm facing here. Is the mobo bad anyway, should I plan on replacing it eventually, or what? Since I'm pretty busy right now, I don't plan on doing anything until I have to. The system works fine except that after I shut down Windows, I have to switch off the standby power switch too.