OK, now I am doing some mild freaking out. The appraiser who was supposed to come today called my broker to say he wasn't going to be here. She and I both interpreted this to mean that he had a scheduling conflict or the flu and needed to reschedule the appointment. So I called him to do that and he told me that he has been told by the lender (who is the one who pays him) to "put the appraisal on hold" for the time being. They didn't tell him why and he really didn't ask as it's not his concern.
Crap. Hopefully the buyers' financing is not falling through. Waiting is so tedious.