Faa' for Fake?

Juan Cole has an interesting post raising questions about the authenticity of a letter, allegedly from Al Qaeda's number two man, Ayman al-Zawahiri, to Abu Musab al-Zarqawi. On the one hand, even on the basis of my admittedly cursory knowledge of Islam, it certainly seems weird that a Sunni would use a form of greeting that invoked a blessing on the Prophet's family. On the other, if even someone like me finds that incongruous, why would a forger include what must, to someone in the region, be an obvious tell? Might a real terrorist include such incongrous bits in a potentially compromsiing correspondence so that it could be discounted as a forgery if intercepted? And how meta can you go pondering the similar sorts of measures either forgers or terrorists might take before you go totally mad?