no more network - but connections
The idea of one big network with computers, servers and other hardware sitting next to each other is finished. You will have to organize your network in zones and qualify your computers with different security-levels and give them seperate vlans, names and identities and securitypolicies. Your network is like an onion, each perimeter an intruder or an internal whacker passes, he will be faced with another one and another one and somewhere somehow he will leave a trace or the alarms will go off.
Place firewalls internally everywhere where needed to protect your zones : Interestingly, Veteran IBM security architect recommended that business use more firewalls to fix the problem of overdependence. Because the "bad guys" can be sitting within the network, he suggested that businesses carve off separate security zones internally to block or limit access to parts of the network http://www.eweek.com/article2/0,1759,1782326,00.asp
another interesting series of articles to read (using ipsec in ms networks for authentification)
A paper that explains how you can seperate your network in trusted and untrusted
computers and use IPSEC to authentificate the traffic and connections
what this means is that you really don't have a network old style anymore but a groups of closed off servers and computers that have connections between each other