I understand that Windows clustering requires Active Directory. If I were to setup a Windows Server 2008 R2 cluster, will there be any issues if the Active Directory server is running Windows Server 2003 and not Windows Server 2008 ?
No there would not be a problem with this. The AD requirement is there so that the hosts in your cluster have a single authentication domain, allowing them to correctly interact with each other. The AD version/mode can be higher or lower than the cluster members.