Naib was down from this past Thursday/Friday night until this evening. The symptom was that turning the power on had no (digital) result; the case fan twitched slightly, and gradually stopped ticking when the power supply was switched off.
I misdiagnosed it as the PSU failing, but it turned out to be that my NIC had failed (no big deal on its own since it was for the internal network only — currently unused). With the card plugged in (in either of two PCI slots), nothing worked; with it removed everything seems fine.
I did learn a good deal about power supplies, including that mine is an ATX-12V (has an extra 12V connector for the motherboard, separate from the main 20-pin connector), and is sized for a micro-ATX case (which is what I have); and that power supplies always have a 5V standby, but otherwise the motherboard tells the power supply whether to put voltage on all the other connectors, and unless you tie the 'power on' pin to ground (with a paperclip) everything will look dead. Also, I had some quality taking-apart time, which may result in cool pictures of shiny objects and cables (and definitely resulted in a broken warrantee-void-if- sticker, but no harm done to the PSU).
The tangential lesson is that, as usual, NewEgg is awesome (insofar as they make it convenent to cancel your order when you realize you don't need new hardware).