Ah yes, it seems like only yesterday. We had just buttoned up the interior of the STI after the installation of our fancy electronic suspension, and we switched on the car to find...a dashboard full of warning lights! Specifically, the Check Engine light, and its friend the blinking cruise control light. (All late-model Subarus blink the cruise control lamp when the Check Engine light is on. I assume this is to make extra sure you don't drive around forever with the Check Engine light permanently lit. And the cruise control doesn't work when that light is flashing, either! Maybe they think THAT will give you the incentive to seek professional service help...)
Anyway, the car was throwing a bevy of diagnostic codes, including "Rear oxygen sensor signal low" and "Neutral position switch signal low" and something else about air/fuel. Clearing the codes resulted in them coming back on instantly upon starting the car, which is a sure sign of disconnected hardware. A once-over of the various wiring harnesses revealed no obvious plugs left hanging, though.
My next stop was to the excellent Subaru Technical Information System web site. There you can pay a small fee ($35 for 72 hours) and download shop manual PDF files to your heart's content, look at wiring diagrams, get detailed troubleshooting instructions, and more. I scarfed up pretty much the entire STI shop manual, plus the full set of wiring diagrams, and got to digging.
I'll post about the resolution to this problem next time.