Let's weave a (completely speculative) web. From a society standpoint we've evolved from nation-states -> feudal systems -> empires -> nation states.
From a technology standpoint we've evolved -> microcomputers -> mainframes -> server/workstation -> upcoming virtualization and web2.0 platforms.
From a communications standpoint we've evolved from military comms -> POTS -> PBX -> Internet Protocol -> Cell networks.
From a newsfeed standpoint we've grown from word of mouth -> printing press -> television -> www.
So if you accept the idea that in the grand scheme of things the trend over time is to become more decentralized then how does that play on security? If "power" is decentralized, whether from king to federalistic systems or centralized switch-based telco hubs to diverse IP based routers, then "power" is also gained by each individual component.
Power is therefore being trusted and redistributed amongst all the systems individual components which allows for larger scaling in what that particular system is designed for. Let's apply this to a corporation. The corporation is designed to make money; and certainly power redistribution to the employees and away from a regimented command structure is happening internally to companies. This is based largely on the technologies the companies use (email,voip,ipods,blackberries,wikis,IM,salesforce,you name it) but also on leadership mindset changes and market forces such as globalization. These are all good things, as the corporation is now more nimble, and quicker at making money.
But it makes security really hard.
What's important? Acknowledging that the network perimeter is a security blanket and not much more than that. Figuring out a way to trust your data and not your infrastructure or necessarily even your data manglers (eg, your employees and partners).
I suspect the walls will eventually come down. Corporate networks will not exist, there will be simply a public (wireless) network cloud with a reliance on private channels and heavy focus on abstraction and virtualization.