See the U.S. Army War College Research Project which describes the threat as being minimal but real enough to require close monitoring for any important changes in that risk.
MAHDI AND THE IRANIAN NUCLEAR THREAT - DTIC
www.dtic.mil/dtic/tr/fulltext/u2/a467203.pdf
USAWC STRATEGY RESEARCH PROJECT MAHDI AND THE IRANIAN NUCLEAR THREAT by Lieutenant Colonel Kurt Crytzer United States Army Colonel Charles Higbee
This paper explores allegations that the ultimate purpose of Iranian actions may be to set the stage for the coming of the Mahdi (or 12th Imam), in accordance with Shiite beliefs.
The author will look at the nature of the threat, define the belief in the Imam's return, focus on possible strategic motives for the Iranian nuclear program and discuss what may be done to counter any threat to the West.
Words in bold above aside, the paper is mostly a political and economic analysis, not a theological one on Mahdi in relation to "Iranian regime" or "Iranian muslim belief" in how and when Mahdi is meant to re-appear. Let's not confuse issues.