I have had problems setting up a cloned server on VMWare with NLB (Network Load Balancing) on Windows 2003 (SP2). The error when trying to add the second host to the NLB cluster is: No interfaces are available for installing a new cluster
The reason for this is that every NIC creates a GUID in registry (HKLM\Software\Microsoft\Windows NT\CurrentVersion\NetworksCards). To be able to set up NLB correctly every NIC needs a uniqe GUID (the settings for NLB are stored in HKLM\System\CurrentControlSet\Services\WLBS).
Solution when cloning servers
Remove the NIC from Device Manager (devmgmt.msc) and do a hardware rescan to add the same NIC with a new GUID.
Unicast/multicast on VMWare
VMWare supports both unicast and multicast but it's easier to implement multicast (this is dependent of how ARP addresses are updated). More information is found in KB1556 in the VMWare knowledge base.