I have been using Skim for a while as my standard PDF reader. After migrating to a new MacBook Air with Mountain Lion (currently running 10.8.2), I can no longer launch Skim from Mail.app.
This same issue was reported to the Skim community ( http://sourceforge.net/tracker/?func=detail&aid=3557462&group_id=192583&atid=941981 ) and closed as invalid, as it was considered a problem with Mail.app, not Skim.
Does anyone know how to get this to work again?