Digicipher and other proprietary systems? I've always been curious as to how Pay-TV and other Conditional Access systems actually stay secure for any amount of time, given that they are usually one way and (I'm assuming) use symmetric keys for encrypting the video/data streams.

Given that the decoder MUST be able to learn the control word keys via some means, and that the keys change so frequently (every 10 or so seconds), it would seem that the only way this and other systems like this are secure is through obscurity.

I've been under the assumption that "security by obscurity" isn't a viable long term solution. So, I would guess that because the key for decrypting the ECM and EMM messages (which themselves contain the key for the control words) must be known by the receiver, the only way that this system would be secure is if they physically secure access to that one key.

How do they keep these systems from being broken by someone just looking at the information in the smartcard that houses the key?

It would seem that by retrieving that one key, the whole system would be broken in one fell swoop, because then you could just listen for all the ECM/EMM messages and get the new keys for a given channel. I don't understand how this is all that secure, but then again, I don't know of a better way other than securing the card. Am I missing anything?
Don't think you've missed something. 


  How do they keep these systems from being broken by someone just looking at the information in the smartcard that houses the key?


Those systems have indeed been (and frequently are) broken via Hardware reverse-engineering. Therefore I would chime in with your "not secure". OTOH: decrypting AV signals isn't like trying to break national security or anything. Multiple black-markets exist where you could (but legally shouldn't!) get rebuild decoder-cards for almost any system and channel(s). 

As indicated, the law is the strongest protection for Pay-TV providers. The signal is merely "crippled" to keep the masses from intercepting and decoding them without additional hardware and (illegal) efforts.