Each physical server running instances of the product requires at least one server license. That means if SharePoint Server is running in a virtual machine, every physical server where that virtual machine might ever run needs a license. In many VMware ESX clusters, virtual machines can move freely between physical servers, so you might need a SharePoint Server server license for every physical server in the cluster.
However, if your SharePoint Server server licenses have active Software Assurance (SA), you may move those licenses freely among servers. Essentially, the licenses can "follow" virtual machines running SharePoint Server around the cluster. This can reduce the number of licenses you need -- you only need enough to cover the maximum number of physical servers that are hosting SharePoint virtual machines at any one time.
Whatever you do, be sure to document it carefully. This is something that will be reviewed in a license audit.
