I stopped up at my FBO at the SYR airport to say hello today and got the story of this incident from an involved source.
The story is that an as-yet-unidentified passenger was indeed smoking in the rear lavatory. This loser, who couldn't even wait the 45 minutes from Reagan National (Washington, DC) to Syracuse, just had to light up. He or she then discarded the butt in the waste receptacle, which was filled with paper towels and tissues. Within minutes (but long enough for the anonymous smoker to return to his or her seat), the waste can went up in a quick, but high flame.
The source went on to say that the Syracuse Police and (IIRC) the FBI were called in to take the names of everyone on the flight and a big investigation is ongoing. This is being treated as a pretty serious crime, as it should be, and evidence such as fingerprints in the lavatory was collected.
Apparently the lavatory smoke detector, which appeared to be free of tampering, never went off until the fire broke out.
Here's hoping the police quickly apprehend the person that put all those passengers' and crew's lives at stake.