FlexWS, the system admin needs to enable "ActiveSync User Management" when editing the domain in question under the "Features" tab.  With this enabled, a domain admin will see "ActiveSync Mailboxes" under Domain Settings.

So the domain admin does not have the checkbox when add/edit a user, but needs to do this on the "ActiveSync mailboxes" section?
Wouldn't the use of the checkbox be more user friendly ?