This isn't sour grapes talking, I've been wrong before and will be again. That said, I want to see the actual OBL video for myself; even more than that, I want to see a video presentation of a professional analysis of the OBL video.
Absent that, I think it justified to regard the transcript alone as a very professionally designed and executed dis-information campaign. And in the best tradition of such, one that works on multiple, self-reinforcing levels of attack. All of which work together to disguise the enemy's actual intentions.
Such a campaign can be effective whether or not OBL is actually alive, of course. Either circumstance can be presented as a positive reinforcement for the jihadist battle position, which can in turn be used to leverage their philosophical position. Rinse and repeat as desired. Those who insist upon regarding the enemy as goat fuckers hiding in caves do so in total disregard of his demonstrated skill at 4th Generation Warfare and small unit urban combat.
If the fey bugger is still alive, I suspect he's mostly been tucked away in the very farthest eastern-most region of Iran these past few years. With a tactically superior combat force, as well as some other equalizing pressure brought to bear, while he negotiates the degree and nature of Iranian involvement in the war (of which the Iraq and Afghanistan Campaigns are only the most visible fronts being contested). The enemy of my enemy, and all that. The Islamist enemy may contest the ultimate outcome between themselves periodically, but the promise of jihad supercedes all else when terms can be arranged.
Assuming the latest video isn't simply brilliant theater, these are my alternative speculations.
UPDATE 9/11/07, 4am CDT: Just for the record, I think this and any other forthcoming "messages from Osama" are a deliberate act of hudna on the enemy's part and that Osama been well and truly dead since late 2002 or early 2003. Just my opinion, of course, your's may vary.