Obviously, a "written statement" is simple to fake.

Wait for the TV appearance. Unless a specific and unassailable gimmick is used to place the date of the appearance (showing a recent newspaper)*, I think we can safely say that bin Laden is dead, and that his followers are trying to conceal it--and not very well.

The propaganda loss to the terrorists of having to admit that the U.S. actually succeeded in punching bin Laden's clock is much greater than any advantages they might gain from showing they can proceed without him, using other leaders.

*Our friendly neighborhood terrorists are not that hip to the sophistication of the Western TV viewer. Watch for an obvious gimmick like showing a pair of unattached hands holding a recent paper. Someone like Industrial Light and Magic could take a year-old movie of bin Laden and easily superimpose today's paper on his shirt, never mind onto a blank paper in his hands, but I don't think the terrorists have access to this kind of technology. Such tricks can also be easily detected.