Microsoft's Lync Server, the successor to the popular Office Communications Server (OCS) is supported in a virtual environment. That said, in order to enjoy the best possible outcome, Microsoft provides dozens of guidance points to help organizations make decisions with regard to Lync virtual server configuration, storage configuration and general virtual host configuration. To help would-be Lync administrators decide whether or not their virtual environments can handle the loads that will result from a Lync deployment, Microsoft has made available a 46 page document entitled Server Virtualization in Microsoft Lync Server 2010. This document goes into great detail about some of the limits you'll face and provides a good reality check.
As you would expect, Microsoft focuses on Hyper-V in the document, indicating, for example, that a virtual Lync implementation can scale to about 50% of the size of a physical implementation due to Hyper-V 4 vCPU limit. Obviously, with the right edition, VMware scales to higher vCPU counts, so it's conceivable that virtual Lync implementations could scale well beyond Lync implementations running on Hyper-V.
This summer, I will be deploying Lync Server 2010 into my VMware environment at Westminster College. I will definitely report back on my experience.