BD+/SPDC is way scary. Here's a good white paper:
http://www.cryptography.com/resources/whitepapers/SelfProtectingContent.pdf
The way BD+/SPDC works is you have a code module on the disc that must be loaded and executed to decrypt the movie, and part of what that code module can do is probe to see if the player's codecs/firmware/hardware have been tampered with, update the firmware of the player, and also uniquely watermark the player output so they can trace the source of a copy.
So for every new release, they can potentially put new and different encryption mechanisms on the disc, new watermarking methods, and new and different hardware integrity checks -- so it's not like breaking DVD's CSS where once you've figured out the one single algorithm you're done -- BD discs could potentially have hundreds of different protection systems, and to make a cracked player work with all of them is much harder than just one.
An analogy on the PC would be like downloading a EXE which is a player for an encrypted video packaged in the same file. As soon as you run that EXE, it flashes your system's firmware, checks to see if you have any software it doesn't like installed, etc, and only decrypts and plays the movie once it's satisified your system is acceptable. If you sat down and reverse engineered the EXE, you could probably figure out how to decrypt the movie without running the EXE, but the next movie you download will have a different player EXE, which means you'll have to crack the next one completely differently.
The flip-side is such a system is probably really fragile, and could quite possibly break in all sorts of nasty unintended ways, locking out legitimate users on legitimate but slightly non-compliant players, which is one of the reasons I understand that SPDC was rejected for HD DVD.
