Thirty days after installing Essential Business Server 2008 your licence restrictions take effect. This means that users are shown as unlicenced in the EBS Management Console will only be able to log into licenced devices (as shown in the EBS Management Console as well). Only licenced users will be able to log into any computer on the network (unless group policy restrictions so limit them).
The licencing enforcement is implemented by the Log On To restriction on the user account. This restriction (on the Account tab of the users object in Active Directory Users and Computers administration program) lists the workstations, by NetBIOS name, that the user can log into and all unlicenced users will have a list of device licenced machines. All licenced users will be set to allow them to log into any workstation. This list is reset at a regular basis each day, but if you are approaching 30 days since installation get your user and device licences correct, don’t miss anyone or any shared device off the list or they will not be able to login or the shared computer will not be accessable to any of the unlicenced users.