I don't know if/how your request can be done to a microsoft IAS, but I have been able to successfully configure Cisco's ACS 4.0 RADIUS server to tie into Windows 2003 AD, and based on User group settings on the ACS server, authenticate and allocate dhcp addresses from different pools.
This provides us the flexibility to have a centralized windows authentication method, and a corresponding dhcp pool each of the equivalent AD groups that are settup on the Radius server.
Now if I could just get the ACS "Downloadable ACL's" to apply to authenticated users I'd be one happy cisco user!
Hope this helps.
-Scott