This article on philosophy of HA is a nice read, in that it gives some perspective about how to do HA.
Traditionally, HA has been done by deploying an application on a set of physical servers and also deploying a layer of HA software like Microsoft Cluster Server or Veritas Cluster Server. This HA layer will monitor and appropriately failover for HA. With the recent trend in virtualization, the application itself will be a VM deployed on a hypervisor. Since it is now loosely coupled with the physical server, the hypervisor layer itself can provide HA monitoring there by taking the HA part away from the application and transparently given by the underlying infrastructure. That is what Virtual Infrastructure is about, from VMWare.
If HA can be decoupled from the application, then why not? It will make life much simpler. I am for it. For VMWare solution to work, we ought to have VMWare HA clusters and VMs deployed on one of the nodes in the cluster. The HTBT business will be taken care of by the virtual infrastructure.
As the author quotes, having VMs AND HA software (like MSCS/VCS) is not definitely worth the effort.