@DCMonkey: No, you can run Windows Server 2012 as a virtual machine on the same physical server as the other virtual machines. I am currently using this configuration with SBS 2011 and it allows me to easily change the hardware in case of a faliure (SBS 2011 supports only 1 NIC so no redundancy there). In this article there is a link to a PDF that confirms that this senario will be supported in Windows Server 2012 Essentials.
As I said before, if you spend the money to propersly secure the network for such a critical application such as email, the license of an extra Windows Server is not the major cost.
