It's a "pillowcase dress." I got the pattern here. It's very basic -- only took me an hour or two. The pattern is simple, but there were some gaps that I had to take a leap of faith to fill in... So far, so good!
Fabric from Fashionable Fabrics. Ribbon from my stash. :)
Oh, and Callie wants me to tell you that the dress is good for twirling: