That’s no longer the case with Windows Server 2012. Both Standard and Datacenter editions will support the same set of features and the same hardware limits. Both will require CALs for authenticated server access. Both will be licensed on a per-socket-pair basis. The only difference will be in virtualization rights: Standard edition can be run in up to two virtual machines (in addition to the host operating system); Datacenter edition can be run in an unlimited number of virtual machines.

via New Windows Server 2012 puts virtualization front and center | Ars Technica.