There are 3 scenarios of MSCS clusters and ESX: Cluster-in-a-box (both MSCS nodes are on the same physical host - great for testing), cross-host (where each of the MSCS node VMs resides on different ESX hosts), and physical-virtual (where one MSCS node is physical, one is virtual).  The requirements for MSCS can change, even in the minor updates, so check the documentation often.  Here’s my compiled list of requirements/tips for MSCS on ESX 4.0:

• You are still limited to two-node clusters with MSCS on ESX 4.
• From a storage perspective, you can use local storage (for cluster-in-a-box) or Fiber Channel (for cross-host or physical-virtual clusters).  There is still no support for NFS or iSCSI (I personally think this is because FC and local storage have more predictable performance - although iSCSI is improving on this).
• If you are doing cross-host, both hosts must be running the same version of ESX (this just makes sense really).
• The MSCS node VMs cannot move as part of HA or DRS.  (HA is being a little redundant for MSCS, DRS is because MSCS is so hyper-sensitive to network connectivity that even a ping loss could failover the MSCS cluster).
• You cannot use MSCS with Fault Tolerance  (i.e. FT VM’s can reside on the same physical ESX hosts, but MSCS node VMs cannot run as FT pairs)
• You cannot vMotion MSCS node VM’s.  (Same reason as DRS).
• You cannot use N-Port ID Virtualization (NPIV)
• If you are using FC and using the native multipathing in ESX, you cannot use round robin as a path policy.
• You must use VM hardware version 7 with ESX/ESXi 4.0 (if you migrated the VMs from ESX 3.5 or before, make sure to upgrade your VM hardware version)
• Failover clustering with Windows Server 2008 is not supported with virtual compatibility mode RDM’s, for Win2008 use physical compatibility mode RDMs.
• You cannot use thin-provisioned disks for the Windows OS vmdk’s, they have to be thick.
• For Win2000 and Win2003 use LSI Logic Parallel as the controller type for the shared storage.  For Win2008 use LSI Logic SAS.
• For physical-virtual MSCS clusters, use RDMs in physical compatibility mode (this just makes sense if you think about it)
• You cannot run storage multipathing software in the VMs or on ESX (i.e. no PowerPath VE).
• You cannot over-commit memory for the MSCS node VMs, set the Memory Reservation option for each of the nodes to the amount of memory assigned to the virtual machine.
• Set the disk I/O timeout to 60 sec. or more (HKLM\System\CurrentControlSet\Services\Disk\TimeOutValue) in the registry.