Microsoft Management Summit Private Cloud
At Microsoft Management Summit 2011, Microsoft implemented a "private cloud" to power all the necessary virtual machines. The infrastructure had to support creating 1,500 virtual machines in under 10 minutes 5 times a day for 5 days in a a row By using the Dynamic Memory feature of Hyper-V, the Microsoft team was able to cut down the number of racks necessary to support all this from 3 to 2. Check out the 2010 and 2011 stats:
You can also check out this video made by Microsoft for some more information.