http://www.cbsnews.com/stories/2003/06/21/attack/main559777.shtml
For more about the Mehdi check Dajjals profile page http://www.freerepublic.com/~dajjal/
David Cook has been writing for years that this "the Mahdi does fit into Sunni religious thought" business is bunk.
Sunnis don't believe in the Twelfth Imam. But the Mahdi is there in the Hadith prophecies.
Sunnis think the Mahdi will be a contemporary Muslim who will establish a worldwide Caliphate. Shi'ites believe the Mahdi will be a 1000-year-old man who will establish a worldwide Imamate. Either Mahdi will kill or convert all non-Muslims.
I suspect that the reporter is a modern, secular Muslim who doesn't know what the prelates are preaching.
As for Abu Haris Abdul Hakim, my first guess would be that he and his army are there to "pave the way" for the Mahdi. If you claim to be the Mahdi, there are quite a few requirements you have to meet -- but if you just claim to be his ally, you can pretty much do as you please.