No, they didn't. The only specifics I've seen on why they concluded it was probably real is the story that claimed the US authorities played the tape to to Ramzi Binalshibh at Camp X-Ray, and he said, "Yip, that's Osama." But, in the words of a famous British call girl, "He would say that, wouldn't he?"
That's what I get for listening to the liberal media - you'd think I'd know better by now. Thanks!