Here's a good one:
"We do know of certain knowledge that he [Osama Bin Laden] is either in Afghanistan, or in some other country, or dead."
Or, just stick with the old classic:
"Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns -- the ones we don't know we don't know."