Wi-Fi Planet outlines wireless security best practices for enterprises.

How to: Define Wireless Network Security Policies - Wi-Fi Planet

Utilize IPSec-based Virtual Private Network (VPN) technology for end-to-end security.

If users need access to sensitive applications from Wi-Fi hotspots, definitely utilize a VPN system to provide sufficient end-to-end encryption and access control. Some companies require VPNs for all wireless client devices, even when they’re connecting from inside the secured walls of the enterprise. A “full-throttle” VPN solution such as this offers good security, but it becomes costly and difficult to manage when there are hundreds of wireless users (mainly due to the need for VPN servers). As a result, consider implementing 802.11 encryption when users are operating inside the enterprise and VPNs for the likely fewer users who need access from hotspots.