First of all, there is some key, let us name it "Official content key". Sony has ENCRYPTION for using to encrypt their games, demos, content and so on. It is not stored inside PS3, only Sony has it.There is simply no point storing it inside PS3. PS3 decrypts this content with DECRYTPION key.
There is "HDD encryption keys" which are different from the ones mentioned above and BOTH of them are inside PS3 (as PS3 decrypts and encrypts this type of content within the system)
Correct me if I'm wrong.
AFAIK, things are a little bit complicated.