Why wait 'til Windows Server 2012 R2
The Windows Server 2012 kernel is shared with Windows-8 which has already been ported to ARM.. I'd be pretty sure that Dell & HP have a skunkworks Windows Server build for testing
The hardware engineers at Dell's Data Center Solutions custom server unit have bent some metal around Calxeda's EnergyCore ARM server processors and donated a box to the Apache Software Foundation so it can tweak and tune the Apache web server as well as the Hadoop data munching stack and the Cassandra NoSQL data store to run on …
One correction is needed from the author of this article. The SL6500 chassis from HP is 4U in height and this means a density of 72 servers per rack unit, as it is written in this previous article on the Redstone servers http://www.theregister.co.uk/2011/11/01/hp_redstone_calxeda_servers/ . This means Dell achieves a much higher density with 90 servers per rack unit. The article presents HP as having the greatest density. You should revise the article and insert the corrected information.