ns-3.12, featuring new support for OS X Lion, is a maintenance-oriented release that also adds a few new features such as fragmentation support for IPv4 and the ability to reuse single-frequency propagation loss models along with the frequency-dependent loss models for the spectrum device and channel modeling framework. Full details are available in the RELEASE_NOTES.

Update: Please note that a minor maintenance release, ns-3.12.1, was made on 2 September 2011 to fix a bug in the Python bindings for the PyViz visualizer.