Sounds like the Mullahs think we are about to capture Osama Bin Laden.
Most Iranians believe the Mullahs of Iran's existence depends on Bush's defeat in November. Therefore they need to help Kerry (who has said he will repair relations with these terror masters.
http://www.freerepublic.com/focus/f-news/1077998/posts By preemptively claiming we have had Osama in custody for a long time they are trying to provide the Democrats with an argument that his "capture date" was a lie and the announcement of the capture "politically motivated."
If we do capture Osama soon, the Mullahs will use this charge of a US conspiracy re: Osama to create doubts about the US on a whole range of issues, most importantly Iran's nuclear weapons development program.
By the way, perhaps the most powerful Mullah of Iran said yesterday: ""All the world-known and notorious terrorists are made with US money, support and experience," he said in his Friday sermon, broadcast live on state radio."
http://www.freerepublic.com/focus/f-news/1087255/posts?page=5#5 So much for Iranian credibility.
There are several possibilities, including the one you suggested. But it may also be simple disinformation to convince the world that Osama is NOT in IRan, where he has spent so much time since Afghanistan was liberated.
It may also be that Iran has made a deal with the U.S. to "arrange" for some of the al Qaeda chieftains to arrive on the Pak side of the border so they can be arrested...there are some deep thinkers in Europe who believe this one.