PS4, patents and other geek stuff

I need the opinion of some hardware geeks.

Recently there has been a lot of conversation around a patent Sony filed that would allow a disc to be tied to a specific console. Rampant speculation around whether or not we will see this in the PS4 persists. Generally filing a patent does not mean that the hardware is forthcoming, but I have an idea on how Sony may try to implement this in the next console.

Game discs basically contain the game itself and some kind of copy protection to make it difficult to duplicate. Game systems have the means to verify that a disc is a legitimate copy usually by decrypting some kind of key.

One way to implement the functionality described in the patent is to design the game discs to be recordable. This would allow the game system to write a unique identifier to the disc (account name, mac address of system or a combination) if the system detects that this is the first use of the disc. On subsequent uses the system would verify that the identifier on the disc matches the identifier on the system.

Some questions that I have:

Is it possible to put data on media like a DVD or Blu Ray and then still write to it?
Is the cost of a DVD/Blu Ray writeable drive comparable to a non writeable drive?
Is this as simple as it seems? If so, why hasn't anyone included this in a system yet?