History has shown many times that there ain't no security through obscurity. If their DRM system has a bug, it'll be cracked, no matter whether it's open-source or not (in the latter case crackers will use tools such as
IDA to reveal the underlying code). If Sony truly wants to convince us and their publisher partners that their system is fool-proof, they better make sure their system is bug-free and perhaps analyzed by some respected cryptologists.