OK, this thread is humming along at a fast clip, so in case this has been raised by another already, pardon my slow reading habit.
There are three possibilities that stand out with regard to the question at hand: either he has recently died, he died prior, or he isn't dead at all.
If we assume Obama... erm, I mean, OSAMA, is in fact no longer among we the living, and if in fact he died a while back, the central question that arises would not be why his death was concealed (answer to that is elementary), but why is he being announced as having been recently killed? What purpose may this serve and who might stand to benefit from it?
It could be a "reelect me 'cuz I be bad" ploy by Osama... erm, I mean OBAMA, but the timing seems far less than optimal for that, so I'd scratch it for now. What does that leave? Chasing Osama has been one of the cornerstone justifications for spending a billion dollars a day in Eye-Rack and Afghan. Killing him off in this manner would seem a bit of killing the goose that lays the golden eggs, would it not? If so, then why would so valuable an asset in the war for public mind-share be retired? There would have to be a very significant reason - but what?
One particularly paranoid notion that crosses my mind is that of the possible utility of martyrdom for those holding the reins of the US military. If we announce to the world that we succeeded in knocking off the sacred Osama, no doubt certain elements of the Muslim world are going to go wholesale native. If I were in charge of all of this mess and if in fact the protection of the people and interests of the United States were my foremost goal, after killing Bin Laden I would swear everyone to eternal secrecy precisely to deny the fanatics their martyr. If, however, my goal is something other than the interests and safety of the American people, making this announcement would be handing them their martyr on a silver platter. This move seems ever so transparently in contravention to anything sensible that I cannot help but question the motives of those making it.
The next question, of course, is "why?" The obvious and perhaps even the correct answer is "power". By martyring OBL, we provide our enemies with an icon that serves to raise and maintain the drive to make war against the infidel. This, of course, guarantees standing, low-intensity warfare punctuated with occasional burst activity for many decades to come. The threat brings the wagons at home into a circle and makes the crushing of any dissenting voice far less onerous in the mind of Johnny Q. Such oppression would, in fact, most likely be welcomed by many because of the false sense of security and order it provides the intellectually bereft nervous nellie.
Of course, it is entirely possible that he really did die just the other day. Any takers?