My student's work in my Open Source Technologies class. She did this in Synfig. I hardly had anything to do with it, except maybe to nudge her in the right direction. Then I had her teach me!
It was another student who figured out how to get Synfig to run on Ubuntu, too. The Synfig from the repositories won't install properly; you have to install the development version from Launchpad.
This class is really turning up a lot of surprises. Some of my students also jumped at Blender and are already deep into it.