One report I read about the data seizure spoke of hundreds of dvds, cds, usb-sticks, memorycards and harddrives that were captured during the raid.
While I am sure OBL and his organization took security *very* seriously I however doubt that such a pile of data is all encrypted in equal thorough fashion.
As some posters already mentioned, it takes only one good fuck-up to compromise everything. I'm willing to bet that in such an a huge volume of data/media there is a fuck-up somewhere.
Also, while I'm also sure his organization had some good it-experts on it's payroll the majority of the people surrounding him, including OBL himself, were probably not very skilled in the use of computers and other modern media/information techniques.
A 54 year old man who spent the better part of his life fighting and hiding in hills and caves.. I wouldn't be surprised at all if he actually *did* write down his password(s) somewhere. Especially considering the fact he lived in the same compound for several years and must have felt relatively safe.
His form of security came in physical form, guns blazing, relocating often (though he fucked up on that one) and trust in the people surrounding him. He understood guns and bullets, not bits and bytes.