I've made this point repeatedly before - now is as good a time as any to repeat it - let's assume, for sake of argument, that the americans faked these two letters, and let's further assume that the letters weren't actually intended to get to their target, but the plan was specifically to 'capture' and publicise the letters.
Question 1: how did the USG think that they could get away with it?
Question 2: where were the denunciations/denials from the purported authors?
As far as I know, 'Zarqawi' never denied being the author of the first letter - despite his many audio releases. And AFAIK, OBL never denounced the letter as a forgery. And AFAIK, Zawahiri never denied sending the letter (although "al-Qaida in Iraq" called the letter a fake - a week after it was released, when serious questions about the authenticity had already been raised).
Are all three of them dead?
"Ayman al-Zawahiri released another tape yesterday, which kind of got lost in the furor over Zarqawi's death. Zawahiri was extremely complimentary towards Zarqawi, lauding him as a champion of the jihad and praising him highly. Quite an ostentatious show, by Zawahiri's standards. "