It seems domain isolation can be used to accomplish, but I'd like a solution that doesn't require IPsec, or more accurately, doesn't require IPsec on the fileserver. IPsec if done in software has a large CPU overhead and our NAS boxes don't support any kind of offload.
The goal is to avoid authenticated users using non-managed machines to access network resources. Network Access Protection (NAP) and the various enforcement points looked promsiing but I couldn't find a bulletproof way to use them [which doesn't require IPsec on the fileserver].
I was thinking when a domain user accesses the NAS box it will first need a Kerberos ticket from AD, so if AD could somehow verify the computer that was requesting the ticket was in the domain I'd have a solution.