Of course, I didn't want to call the plumber over a holiday weekend, so we stalled until Monday when we put in a call to the plumbing service that we usually use. They rodded out the drain pipe from the upstairs bathroom, which was what was needed to fix the problem.
This required removing the toilet. I had asked daisy_knotwise to tell the plumber that he needed to use an extra-thick wax ring when reseating the toilet, because we had installed ceramic tile on the bathroom floor that raised the toilet just high enough that a standard thickness ring wouldn't do the job. (As I'd discovered when samwinolj and I popped the toilet downstairs off in order to chase something that Julie had dropped down the toilet. Fortunately, we figured this out immediately, so the only problem was the extra trip to the hardware store to get the right ring this time.) The plumber assured us that he had put in a new ring and caulked and that we shouldn't have a problem.
So later that night, I was sitting looking at the old stain on the ceiling of the breakfast nook that had been left by previous adventures with this toilet. (Yes, we will paint again some day, but not until the girls are just a bit older.) The stain looked suspiciously larger and darker than I recalled. This seemed bad.
Since Katie and Julie were upstairs playing on the computer, I had the chance to grab a ladder and reach up and touch the stain. It was wet, which strongly suggested to me that we had a nice new leak.
I called the plumbing service back and they agreed to send someone out in the morning. (Which was fine, since we still had two working toilets.)
There is now a thick wax ring installed under this toilet.
I'm hoping -- and expecting! -- that the leak is now stopped.