souljawebb said:
Actually, it's been stated that the background in the Deathwatch rulebooks is canon, though I can't find the reference just at the moment. It's all approved by GW long before publication, and FFG have been given the opportunity to define the history, origins and structure of the Deathwatch.
No, an RPG published by Fantasy Flight doesn't suddenly become canon just because they produced it... but it does become canon because GW's IP department have approved it.
It's all canon. Whether or not you agree with it, or choose to accept it, is an entirely unrelated matter.