Where can I find out more about Network Access Protection?

The best place to start is the Network Access Protection (NAP) "landing page" on the Microsoft website.

NAP is a great new feature of Windows Server, Windows Vista and Windows XP Service Pack 3(due soon) which enables you to measure the policy compliance of client machines and determine which resources they can access based on the result.

I wrote a summary of NAP on my recent post what is the best security feature of Windows Server 2008.