Response:

This was a frequent accusation of U.S. military commanders in Iraq during the Bush Administration.  Such allegations are heard much less frequently now and the scale of Iranian involvement in such attacks was probably exaggerated at the time.

Any support for militias that attack U.S. forces is obviously inexcusable and unacceptable, then or now.  The hard fact is, nations seldom do nice things for those they perceive as their enemies.  Witness our support for Afghan mujahadeen against the Soviets in the Eighties.  During most of the period in question, the Bush Administration kept Iran in constant fear of bombing or invasion, while limiting its "diplomacy" with Iran to occasional meetings in which to air U.S. accusations of Iranian meddling.¹ 

Iran's role in Iraq is one of the factors that the Obama Administration must consider as it develops its overall approach to Iran on a range of issues.  Iran is obviously much more likely to play a constructive role in Iraq if included in consultations, than it will if it is excluded.

Footnotes