It's a dangerous game to be playing here, supposing that any allegations against Iran are merely propaganda and, like the information (allegedly) untrue that supported war with Iraq, subject to inherent doubt. There have been reports of Iranian involvement in Iraq for years, not just months, as anyone who's followed the story would know. And it is, of course, entirely possible that Iran is escalating its involvement, believing that the US is close to being politically forced out of the Iraq.
Stories like the one linked above leave the US defenseless, because any threat that might arise suddenly becomes another "Iraqi WMD lie." One sees it all the time from the Left, in responses to accusations such as these against Iran to reports of planned terrorist attacks (or, even when terrorists are caught in the act). Suddenly, the US is unable to defend itself, because no evidence that it can provide for doing so is acceptable.
Except, maybe, the smouldering ruins of an American city. And maybe not even then.