I had thought of this just a couple minutes ago, so I opened my case and booted the computer, and sure enough no mobo activity. Everything that wasn't linked to the mobo was powering up no problem. I disconnected and reconnected every cable, with no luck. So while the computer was on, I messed with the cables and jiggled the bulk of them up near the PSU, and bam, power. So I guess I've got a bad, loose cable in my PSU. I'll be replacing this ASAP.
Problem solved!
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