Bruce Schneier summarizes in a blog post a disturbing topic regarding a New York City locksmith selling “master keys” on eBay, providing access to various services such as elevators and subway entrances.
A discussion about this blog post on Hacker News yielded some interesting conversation, including an even more disturbing article describing how simple it may be to create master keys for a set of locks given only the lock, its key and a number of attempts.
I'll let you ponder the implications of both of these topics. Here's something to get you started: organized crime could use these keys to effectively evade law enforcement or break into millions of “locked” homes. Crackers could gain intimate access to various city systems whereby they may be able to further obstruct or infect systems. A security system is only as strong as its weakest link. Keeping citizens in the dark about these issues gives them a dangerous and false sense of security.