This last tape doesn't prove that OBL is alive because he mentioned the possible bombing of Al-Jazeera, suggesting to some that OBL is aware of current news. I don't believe that something as significant as talking about bombing the news station could have been kept secret at the time.
I think he was killed at Tora Bora. Even the fact that Peter Bergen, an authority on OBL, can't tell either way, is evidence that he's dead.