Why would Osama use porn to hide secret messsages? I think one of the benefits is free porn is so actively sought by many people around the world. So one can easily use bittorrent or any other distributed file sharing networks to distribute the material anonymously. And because bittorrent you don't download from one source, but bits and pieces from different people, who then in turn share their copies so other people can download bits and pieces, it will be very difficult to trace the download back to the original distributor.
As with any encryption system, you need a system of secretly passing the key. In normal internet encryption, one would use a 3rd party like Verisign. However, for terrorist messaging, you cannot have your encryption keys out in the open. So what you can do with a porn scheme is identify the porn star that's in the videos, find the original video/picture from a pay site. Then download the original video/picture from that site and use it to decrypt the message.