The RADIUS Server software is there but it only shows up if an AirPort Extreme (or presumably TimeCapsule) is on the same network and in factory default condition. If you have previously setup the WiFi on the TimeCapsule then it will be ignored and the (hidden) RADIUS option not show up.
While the RADIUS server in Snow Leopard Server was itself limited to just supporting Apple devices it at least was more flexible in what AirPort configurations it supported. With Lion and Mountain Lion it is now limited to just a single configuration and at least as far as I am concerned dumbed down in to completely uselessness.
As the RADIUS option is using the standard FreeRADIUS open-source software you could manually configure it and have complete control. This is also possible with non-Apple devices. There is also a utility available on the App Store for Mac to do a simple RADIUS setup with more flexibility than offered by Apple.