OK. Let me give you an example.
The guy decides to really secure his network. He disables broadcasting the SSID, and implements WPA2 encryption. This, however, provides access only to a small non-Internet connected mini-network with no DHCP and one machine, identified by IP address only, which only responds (ie no pinging) to a specific IP address using the ports needed for an IPsec VPN. The VPN, in turn, uses 1024 bit blowfish encryption and requires a SecureID password to initiate the connection. The VPN provides access to the outside world, but requires you already know the IP addresses of the DNS server and default router, none of which are in an easily guessable netblock.
The system is also set up so any unauthorized activity results in the outside connection being dropped (ie any attempts to guess the passwords result in the wireless router shutting itself down. And, just to add that extra special extra layer of security, the owner switches off the entire system when he leaves to go on vacation.
So an elite hacker comes along and has access to the network within thirty seconds. How does he do it? How does he circumvent all these security measures?
Answer: he throws a brick through the window, enters the house, and HOOKS HIS LAPTOP UP TO THE DSL MODEM.
Virtually every security device can be circumvented.