If Osama isn't dead, he could easily prove that he is alive with another tape, with him holding a copy of a very recent newspaper.
Furthermore, Obama confirmed that Osama is dead on national television and tied his name to it. If it turned out that Osama was still alive, it would ruin Obama's career. Obama is smarter than that.
This is true. Still, I think it's highly possible that they killed him (or at least he died) ages ago they just didn't tell anyone till now.