Ok, so first I tried using LDAP. I could get my 1512 to attach to LDAP server. Now what? There does not appear to be anyway to work with domain user information. Sure, I can give permission to a domain user to access a shared folder. However, how do they access it? I gave two users r/w access, but they cannot map a drive nor login via a browser using there domain information. Disconnected from the LDAP and joined the DS to the domain...same problem.
So what exactly does Joining the Domain or using LDAP accomplish in DSM 4.0? It appears to be worthless in DSM 4.0. Unless there is some hidden secret that was left out of the documentation, it appears that Domain/LDAP usage is nothing more than a pipe dream.
2012-03-29 Update: So, LDAP is bust on a Microsoft domain. Joining the 1512 to the domain does work, however it took over 32 hours after connecting before I could do anything with it. Now I am able to assign domain users to resources and they can connect.