Does anyone cook/eat bladders? There's a few places in the world that really don't waste any part of an animal when it comes to cooking, but I've yet to see any form of cooked bladders. 

Is it an item that is cooked -or- eaten anywhere in the world?
Yes.  Here's a specific example.  Asian cuisine also has a strong tradition of using a wide variety of organ meats, many of which are considered delicacies.

The bladder would be a particularly tough piece of protein to prepare, but you can make just about anything palatable with proper cleaning and a long braise.