The first DLC for Bulletstorm - Gun Sonata - is out on PSN and Xbox Live. It contains three new arenas for the Anarchy mode and two new levels for the singeplayer Echo-mode. There's also two brand new leash-attachments, Flamingo and Pulp. The PC-version, according to Epic, should be out "soon".
And speaking of Sonatas - Epic has made the whole Bulletstorm soundtrack available for free, just swing by their community website to pick it up.