I recently wrote a lot of documentation about the Google-Authenticator, how to use it with PAM or RADIUS and how to implement a Freeradius-Proxy to be able to use Google-Authenticator in addition to your current RADIUS setup.

Feel free to take a look at my public Wiki to read more about:

• Google-Authenticator
• FreeRADIUS Proxy
• openvpn-als (aka Adito)
• Webmin custom module for Google-Authenticator
• …

Here are some nice screenshots to the above contents:

Google-Authenticator iPhone App

Custom Webmin Module for Google-Authenticator

Different Windows Clients