The AirFortress, from Fortress Technologies (http://www.fortresstech.com) is a comprehensive layer 2 security solution for wireless LANs. It was the first US Government approved WLAN solution, and they have sold it to the US Army and the Veteran's Administration. They have clients for Win32, Pocket PC, Palm, and DOS, and can authenticate to RADIUS, NT/Active Directory, and their own policy server. They also feature "device authentication", which they say can allow admins to limit what laptops/pda's can be used by valid users.
Here are some articles from their web site.
Keenan Systems now is an authorized AirFortress reseller and can answer detailed questions on the product line.