This is it:
In almost four years of parenting, I don't think I've ever seen one of my children lay down in the middle of the floor and fall fast asleep. It was only about 15 minutes before naptime, so I just moved her to her bed, where she slept soundly for the next 2+ hours.
She woke up with a fever.
Aha. Mystery solved. So we had to cancel some fun plans for tonight so that we won't expose anyone else to whatever Maggie has. Hopefully it's just a little 24 hour bug that will pass quickly. At least she's getting this over with before her birthday this week!
In the meantime, while Maggie is getting over her little fever, the Roebucks are holing up for the night with the doors and windows open on this gorgeous August evening. We're renting 'Enchanted' for a family movie night and crossing our fingers that no one else will get sick. :-)