For the past week or so there has been a flurry of activity on a libMesh development branch updating the core data structures in the library to take an optional communicator argument, as a step for allowing e.g. a mesh or set of physics which is represented only on a portion of the available processors during an MPI job. We've tested this with a number of application codes and everything that used to work still does, but this touched some 6,800 lines of code, so be aware if you are following libmesh/master that next time you do an update there will be a big change coming.
The API should be preserved, but as always please report any compilation or runtime issues you may encounter!
-The libMesh Development Team