Are these application servers? You would be looking at clustering, which is used very often with IIS, SQL, file server (DFS) and Exchange by a large number of Fortune 1000 firms to achieve 99.9% uptime or better.
The technical overview of Windows 2003 clustering is available here:
http://www.microsoft.com/windowsserver2003/techinfo/overview/clustering.mspx
As cluters are a mixture of both hardware and software, even when virtualized, I am not quite sure what information you are seeking. You should note that while clusters can be virtualized that applications that make heavy usage of disk operations (SQL, Exchange, File Servers, Print Servers, etc) do not typically do well in a virtualized state. In addition, there is typically a 25-40% hit on processor utilization from the virtualized state.
Individuals who say that clusters are not a proven technology obviously are lacking in a depth of experience in working in data centers. I would advise you to not place much emphasis on their commentary.
One of the questions that I am sure you will have is "Can Active Directory be clustered?" The answer is that Active Directory cannot be clustered, but a node of a cluster can be an Active Directory domain controller, but it is not recommended by Microsoft. From my personal experience, I can tell you that it is not a great idea, due to LSASS causing potential issues with servicing LDAP requests and consuming all processor resources on the cluster and causing the cluster to fail.
Suprisingly, Active Directory does quite well on a virtualized server. Several organizations use this technique to assist with their disaster avoidance compliance plans and set up virtualized DC's to come online in the event of a disaster.
Best of luck with your research.
