So the ABS sensor on my wife's Toyota went bad. I poked around online and found that it is sold as one unit with the rear wheel hub. I looked more closely at the sensor in question and noticed that it had just fallen out. Before dropping $150 on a new hub to install, I used some J&B Weld to stick the sensor back in, and it's been working fine since...no light, and I slammed the brakes from about 30 MPH in an empty parking lot (thanks to Sandy) and everything buzzed and anti-locked perfectly.
Yay for a $1 (5 minute) fix, instead of a $150 (2 hour) DIY bill, or a $750 (half-day) shop bill.