In general we have virtualized almost all of our Windows servers except for about 10%. We do have 3 prod SQL Server clusters that are not as busy as some would think or as large as some would think. When it comes time to replace the hardware / upgrade my boss and I are in agreement to create VMs for these. SQL Server wise all of our standalone non-clustered SQL Servers are now virtualized except for one which will will do in the near future.