I don't know if this will make you feel better or worse, but...
I've been watching the nest out my office window most of today. An adult robin has come by a few times and hopped in the nest before flying off again. I don't know if that was one of the parents or if it was an entirely different bird, but it does not seem to be completely abandoned quite yet.
Maybe the parent was looking for the babies -- who are all gone now. Or, maybe, they're checking on the place and planning on having another brood there in the next few weeks.
Either way, I'm going to leave the nest alone for a while.