Microsoft first announced on October 10, 2005 that Windows Server "Longhorn" Datacenter Edition would give customers the right to run an unlimited number of virtual instances on a physical server. At the same time they said that with a single license of Windows Server 2003 R2 Enterprise Edition, customers could now run up to four virtual instances. You can access the full press pass here.
That was a significantly good announcement for customers. Those who use Windows Server 2003 R2 Enterprise Edition would get to try the benefits of virtualization. If it became necessary to run more than four instances they could eventually give into temptation and look for an expansion by purchasing the Windows Server "Longhorn" Datacenter Edition. Good strategy!
On July 12, Microsoft decided to improve this offer and shock the customers a bit more, and then some more. Starting October 1, 2006, new servers licensed with Windows Server Datacenter Edition would be entitled to run an unlimited number of virtualized Windows Server instances, including Standard, Enterprise and Datacenter Editions. Multiply them and mix them up as you wish.
This is not the only change made by Microsoft in order to promote their Windows Server Datacenter Editions. Want to know what else they did? Read the full announcement here.