All releases of AutoCAD for Mac stopped working with the recent update of Apple's OS X operating system, version 10.8 colloquially known as "mountain lion." (These sorts blockages of major applications of just don't seem to happen with Windows updates.)
Today Apple released a half-gigabyte-sized update 10.8.2 that adds Facebook integration (boo!), but also fixes all that ailed AutoCAD for Mac. It now runs again.
(The readme makes no mention of the AutoCAD fix, true to Apple's secretive way. Maybe the flaw was just an embarrassment for both Autodesk and Apple.)
Rav of UK reports, " 10.8.2 did not fix the issue on my macbook pro as AutoCad is still not working."