I updated two of my FlatPress themes tonight in order to make them 100% responsive. They now render properly on any kind of mobile devices, which is both useful and cool.
Grey Matter: https://github.com/MarcThibeault/greymatter
I'm currently fooling around with the default FlatPress theme: Leggero. I expect to be able to come up with a version 2.0 that will look incredible.