A Professor at my University was asked his opinion in U.S. History class once. This Professor was former counter-terrorism/counter-intelligence in the US Army, and earned his doctorate after his service.
I remember him clearly stating that IF bin-laden was dead and the US gov't knew it, we would not publicize it. His reasons were, among others, that it would provide an advantage to the other side...in that they would attempt to wreak havvoc b/c of their slain leader etc...
I tend to believe that theory...OBL is dead, but it is not being publicised.
Holly just my .02