They try not to broadly publish the numbers but I've heard that about 33% is the current graduation rate for a BUD/S class -- this may NOT include rollbacks though (or it may) -- so 33% might be understating the percentage chance of any one individual to complete BUD/S (if it just looks at finishing "on time" with your class and doesn't count rollbacks).
Friends in the SEAL community have confirmed that (a) the overall graduation rate for officers in BUD/S is significantly higher than for the enlisted guys; and (b) even for officers, the USNA graduation rate is significantly higher.
On the first point, my friends generally think it's a combination of factors: (1) officers in BUD/S trend older than enlisted, so they often have a better sense of what they want and are better able to resist distractions during BUD/S; (2) it is more selective to get an officer slot; (3) officers know it is pretty much "one and done" whereas enlisted guys know they have a better chance of going back into the Fleet and getting another shot at BUD/S down the line; and (4) the USNA guys have great preparation and that drives up the overall officer success rate.
Specifically looking at number (4) above, as Hurricane referenced, USNA not only has a good screening process (lots of PT monsters learn they actually can't stand immersion in cold water during the SEAL screener, it turns out) but then preps the guys big time before they head to BUD/S.
I think 90+% is high -- there's still the injury risk -- but from anecdotal accounts of USNA guys on their BUD/S experience, I could see the USNA rate as being around 70%. Still just guessing on this, though.