Wireless LANs have major security issues. Default installations of most wireless networks allow any wireless NIC to access the network without any form of authentication. You can easily drive around with laptop in hand and pick up many network connections. War driving is quickly replacing war dialing as the method of finding back doors into a network. Wireless LAN administrators probably have no idea that their network is accessible to anyone with a wireless NIC.
Wireless ISP providers must be very conscious of their wireless network configuration. If someone is able to access the network without authentication, that person is essentially stealing service.
Once a user gains access to the wireless network, whether authorized or unauthorized, the only thing keeping that person from accessing unauthorized servers or applications is internal security controls. If these are weak or nonexistent, an unauthorized user could easily gain access to the network through the wireless LANand then could gain complete control of the network by exploiting internal weaknesses.
Denial-of-service attacks are also a very real threat to wireless networks. If you're running a mission-critical system on a wireless network, an attacker does not need to gain access to any system to cause damage and financial harm to an organization; he just needs to flood the network with bogus radio transmissions.