As per my 70-290 text :
"By default, Windows Server 2003 domain controllers are configured to accept terminal services connections only from members of the Administrators group. Even users you have explicitly added to the Remote Desktop Users group are not permitted access. To override this behavior, you must change the effective value of the Allow Log On Through Terminal Services group policy, which lists the Administrators group only, by default. To do this, you can either modify the domain controller's local computer policy or define this same policy in the group policy object (GPO) for an Active Directory object containing the computer, such as Default Domain Controllers Policy GPO."
Therein lies a new issue - if I give our Terminal Services customers Administrative access to a domain controller, wouldn't there be the possibility that someone who knows just enough to be "dangerous" would do something to break it? If I modify the Group Policy, then our REAL Admins won't have the permissions they need either...