There is a small extra loss for the VM Server. However compared to the cost of the extra hardware needed to host the virtual machines, I've found that the loss is acceptable.
*** Improved by Wrobinson on 4/18/08 ***
The loss can be negligible or unberable depending on the configuration of the VM hosts, as well as the operational profile of the VM guests. There are different objectives for going virtual, some of which include consolidating assets, in which case, you will be interested in achieving the highest pack ratio possible. Alternatively, you may be looking to enhance business continuity and disaster recovery capabilities, in which case, the pack ratio may not matter as much. There is yet another possibility and that is archiving where storage will be the primary concern.
There is an old saying that there are several ways to skin a cat but I am a cat owner, so I am not interested in how many ways there are to do that. The saying is true, however, when it comes to virtualization. You need to come to identify your objectives which will in turn help determine the best design principles and practices to best achieve those objectives. If it is performance that you are after, then make sure to factor in processing, memory, fast disk drives and separate disk controllers.