I like the diagram Mark O’Neill of Vordel put in a recent post, “Identity is the New Perimeter.” That phrase has been floating around for some time, but I think this diagram illustrates the concept in the simplest, clearest way I have seen:

The article does a good job of describing this new way of looking at security.  As Mark mentioned in the post, Bill Gates once said, “security should be based on policy, not topology.”