It's One LAN
Security is probably the biggest concern in enterprise Wi-Fi deployments. The security standards are weak, but are getting better. IT departments need to take a holistic view of network security which includes Wi-Fi specific policies that balance usablity with security. But the other thing to remember is that while the existing Wi-Fi security mechanisms (WEP, closed networks) are vulnerable, you are still more secure with them on that with them turned off.