His letter has been made public online over a period of days in four parts here (note that it isn't until the third part that readers discover that the writer is Haggis):
As anyone who watched Tom Cruise jump up and down on Oprah's couch knows, Scientology (AKA, The Church of Xenu, alien emperor) actively courts celebrities, having a "Scientology Celebrity Center" in L.A. dedicated to pandering to their egos. They also have a history of aggressively attacking those who dare to speak out against them. While I'm not a big fan of Mr. Haggis as a filmmaker, his heart has always seemed to be in the right place and I respect his going up against them so publicly.
It will be interesting to see how the "church" responds.
Update: The letter can now be found in its entirety here.