Hello and welcome to the forums.
Note: This thread was moved from the VMware Server 1 forum to the ESX 4 forum.
What kind of cluster are you looking to run? MSCS is officially supported. Outside of this, you may want to check with the software vendor.
Good Luck!
If the clustering is MS clustering then only active passive is supported.
I dont see why you couldn't run an active/active cluster.
I'm no expert on clusters by any means, but we are running an active/active one on physical servers now.
I don't see any configuration or shared storage issue that would prevent it from being a virtual setup.
So long as your SAN is correctly setup and the disks presented to the Cluster/OS I don't see any issue.