You have probably seen the news coverage of the allegations made by defectors from Iran's intelligence service that Iran's government not only had advance knowledge of the 9/11 attacks, but actively assisted al-Qaeda in that effort. The May 20, 2011 New York Times reports on a $100 billion civil suit against Iran, asserting this based on sworn testimony from former employees of Iran's intelligence service.
My first reaction is that this would not be a surprise. The 9/11 Commission report made it clear that there were serious unanswered questions about Iran's involvement in assisting al-Qaeda both before and after 9/11. Iran hates us (in case you haven't noticed).
My second reaction is that there are people in Iran who have a strong interest in seeing us knock these thugs out of power--and are probably not adverse to telling us a plausible (although perhaps inaccurate or incomplete) story to get us to take action to that end. Remember that at least part of the problem with the WMD story in Iraq was not that "Bush lied," but that Iraqi defectors with a strong interest in seeing us overthrow Saddam Hussein told us plausible (and who knows, perhaps even accurate) stories of WMD development.
I would like to know more. I think it is likely that if this story is a pack of lies, the Obama Administration would not be interested in keeping it alive.