I must take exception to this statement:
"This would weaken the United States, and put the hurt on Iran, an arch-enemy of al Qaeda."
That is just not true. The Iranians and the Zarqawi wing of AQ may not have liked eachother, but Iranian support for AQ has been documented for years. It was first discussed in "Breakdown" by Bill Gertz. Then the September 11th Commission Report detailed Iranian ties to and support for Al Qaeda. Two recent books by Richard Miniter and Curt Weldon also provide indications of Iranian cooperation with Al Qaeda.
I admit that this is confusing. It might be a goal to somehow incite a war between Iran and the United States. If anything, it seems that it would solidify ties between Iran and Al Qaeda, with both sides trying their best to cause massive U.S. casualties.