I was recently working on a customer site where they used Virtual Server 2005 for their test environment (excellent!). They had a production Exchange cluster but had not simulated in test because they believed it was not possible to recreate in Virtual Server. I remembered seeing some sessions at TechEd on the subject but had never actually done it. A quick google gave us step-by-step instructions on how to get this done.
Clustering in Virtual Server 2005 makes use of the virtual SCSI adapters. You set node one to ID 7 and node 2 to ID 6. Then create a fixed size virtual disk and set it to use a shared SCSI bus. Connect both virtual machines using SCSI bus 0 and then you can implement your cluster as you normally would.
The client was well impressed with the virtual Exchange cluster they had at the end of the engagement.
I’ve done some googling and it appears you can do something similar with VMware Server by mucking around with configuration files. I also found this on the VMware site but I haven’t tried it yet.