Hey - thanks for the sympathy and for keeping eyes peeled. It's posted on craigslist too, and a few other places I've thought of.
It was locked up in Harvard Yard, using a cable lock. Ordinarily I wouldn't feel OK using just a cable lock, but I thought the security guard 10 feet away would have been some protection.
It's now turning out to be a possibility that Harvard took it. They're apparently clearing out their dorms and sometime (they won't say when) they're confiscating bikes that are "abandoned" on their racks. If by "abandoned" they mean locked up over the weekend, they might have taken mine. If it were a thief, I'd expect to find the broken cable on the ground, but everything was removed cleanly. The half of the rack where I was locked up was completely cleared of bikes, even though it's usually packed. I just spent a very long time on the phone being shuttled around from person to person, none of whom could or wanted to help. I'm not a student, I just sometimes lock up there when I'm doing errands or getting the T. I know I'm not the only non-student who locks up at this particular rack, which is right by the bus/T.
So general warning: if you're around Harvard, don't lock up to their racks this week.