I do not believe that Iran is likely to be aiding Al Qaeda.
Al Qaeda has repeatedly called the Shia traitorous heretics. It has destroyed major Shiite shrines and killed thousands of shiites in Iraq. I think that the hatred is too strong for an alliance under present circumstance.
But, you are right in saying, "The fruit cakes leading Iran are capable of just about anything." If they ARE aiding Al Qaeda to destroy Shiite shrines and people in Iraq, sooner or later it will backfire on them with their own people.