I even think I remember AICN themselves reporting on this earlier, but can't find any mention of it. Nevertheless, Stephen Fry has been attached as a writer for years now.
This comes from the Lincolnshire Echo (Lincoln) on Mar 14, 2008:
Comedian Stephen Fry has finished his second draft of the screenplay
for the new film about the Dambusters raids.The entertainer and
best-selling author made the announcement to his fans in an Internet
podcast.
http://www.britmovie.net/britforum/latest-cinema-attractions/3055-dam-busters.html
And the script is mentioned as being already written in this online article from 2008:
http://www.stuff.co.nz/entertainment/510913
The news about preproduction might be new though, so that's good news! Been looking forward to this one...Ā