I actually would find it more likely that OBL died from kidney problems than from bombs at Tora Bora. But most of our intel folks firmly believe he's still alive. They have access to much more info than we do, and it simply makes more sense to me that the coward wasn't around Tora Bora when things got ugly.
The "tin-foiler" in me thinks thak our spooks are happy to maintain that piece of fiction for a couple of reasons. First, for domestic consumption, it keeps the lefties amongst us from ending the WOT. Second, the continued life, however fictional that life may be, of OBL keeps the rest of AQ from going completely underground. The combination of these two things, plus some luck, make it a whole lot easier to kill and detain more AQ's.
[/tin foil]