Various IPSec implementations support the use of certificates and other forms of strong authentication. Beyond this, you should place a firewall between the wireless network and the rest of your network. Unless a user authenticates properly, they should be contained to the wireless network, where they can do less damage. A system similar to this is used by the University of Alberta for its public Ethernet networks, as described in a paper by Bob Beck. However, this still allows the attacker access to others on the wireless network, so end security on user machines is still important.
Wireless networks are inevitable.
802.11, Bluetooth, and others are coming. They all in general have poor security models and flawed implementations. Relying on wireless features such as frequency hopping and spread spectrum radio is also not ideal, as the same consumer equipment can usually be used to monitor it. But even if this goes as planned, an attacker will still be able to attack your wireless users. Hopefully the next major wireless network protocol will be done correctly.