Just wanted to get this out there as it’s a revamped whitepaper. Get it here. Pay special attention to the mixing physical and virtual servers section.
This is somewhat confusing but it states:
Generally physical and virtual servers can be combined in a deployment, with the following limitations:
Mixed virtual and physical servers within the same pool are not supported. A pool of Lync Server roles must be deployed as a homogenous set of servers (either physical or virtual) which roughly provide the same amount of resources. For virtualized deployments, ensure that the virtual machines are hosted in a way that equal resources are available to each guest in the pool. Lync Server does not perform load-based load balancing with the exception of the A/V conferencing workload. Note that a virtual Front End Server and a physical SQL Server Back End Servers are explicitly supported.
Balanced end-to end-performance is required. When sizing a virtualized Lync Server deployment, use the guidance provided in this document to size each role adequately, and perform tests using the synthetic load tools available to validate that no bottlenecks exist.
 A pool is defined as a Lync Server pool with more than two servers including the follow Server roles: Enterprise Edition Front End Server, Director, Mediation Server, A/V Conferencing Server, and Edge Server.
So if you have a pool of 4 front ends you can not have 2 virtual and 2 on physical servers. They must be all one or the other. But you can have 4 virtual frontends and a physical SQL backend. So the opening statement can be a little misleading. Also of important is the last note. A Lync deployment can be more than one pool . You can have a pool of mediation servers or AV conferencing servers etc. So while in the same pool you can’t mix virtual and physical you can have a mix of pools. So your frontend pool may be on virtualized servers but your AV conferencing server pool might be physical. Like I said it’s a little confusing but hopefully that explanation helps. Of course it may seem obvious but if you are collocating AV conferencing servers and mediation servers on your frontends you can not mix those as they would be in the same pool as your frontends.
Being familiar with Lync terminology and reading this statement several times is very handy.