If you tried upgrading XBMC under OS X, run a MythTV backend, and get the error “No PVR Client has been started yet”, see this thread

This thread contains information on the buggy XBMC version 12.3 for OS X, and a couple of suggestions for reverting back to XBMC 12.2, which will fix the problem for now.  This issue does NOT appear to be present in Linux versions of XBMC 12.3.