Deepak,
Excellent question. I'm currently deploying a test environment for a large scale wifi network.
You can see more project info at wiki.socalwifi.org and at socalwifi.blogspot.com.
I am documenting everything we are doing and will have a Blog post up soon regarding our kerberos/radius/VPN deployment and how we load test/scale it.
i have yet to see any existing information on the back end components specifically related to muni wifi.
I would imagine standard documentation/white papers related to scaling a large enterprise wifi network apply.
Charles