Let's consider the possibilities.
The letter is real: If so, does it matter? Isn't this pretty much what we would expect them to say?
Things may develop faster than we imagine. The aftermath of the collapse of American power in Vietnam-and how they ran and left their agents-is noteworthy. Because of that, we must be ready starting now, before events overtake us, and before we are surprised by the conspiracies of the Americans and the United Nations and their plans to fill the void behind them. We must take the initiative and impose a fait accompli upon our enemies, instead of the enemy imposing one on us, wherein our lot would be to merely resist their schemes.
The letter is fake: If so, this raises the question of who wrote it. al Qaeda claims it was the Black (White) House. I don't think this possibility can be ruled out, but others could have faked it as well to provoke this sort of incident. If the White House did fake it as a means to drum up support for continuing the occupation of Iraq, it brings Bush another step closer to impeachment.
However, I doubt the Bush administration would try this particular stunt right now when there is already too so much public scrutiny of whether or not they fabricated the reasons for going to war in the first place. Maybe that's giving them too much credit.
Ultimately, does it really matter? Using a letter allegedly from al Qaeda as a justification for your foreign policy seems monumentally stupid. Plus, it seems unnecessary. While I believe going to Iraq in the first place was a terrible idea, we also cannot just cut and run. We went in, destroyed their infrastructure and defenses, and until we make them self-sufficient and able to defend themselves from hostile neighbors (Iran, Syria, insurgents), we can't leave.