Microsoft NAP and multiple domains

Do you a RADIUS server that can authenticate to several Microsoft domains or other RADIUS servers?

RADIUS Proxy: "Network Policy Server (NPS) can be used as a RADIUS proxy to provide the routing of RADIUS messages between RADIUS clients (access servers) and RADIUS servers that perform user authentication, authorization, and accounting for the connection attempt. When used as a RADIUS proxy, NPS is a central switching or routing point through which RADIUS access and accounting messages flow. NPS records information in an accounting log about the messages that are forwarded."

Network Access Protection that comes with Windows 2008 can do just what you want. Read more here