Hot off the press….
Overview of Hyper-V: http://technet.microsoft.com/en-us/library/cc816638(WS.10).aspx – Updated Feb. 10, 2010
What’s New in Hyper-V: http://technet.microsoft.com/en-us/library/dd446676(WS.10).aspx - It does not show an updated date, but it does now have R2 info in it.
Getting to Know Hyper-V: http://technet.microsoft.com/en-us/library/ee256064(WS.10).aspx - Was already updated for R2 on December 15, 2009.
Overview that was included in Help in Hyper-V 2008 R2 http://technet.microsoft.com/en-us/library/cc770901.aspx
Overview section of deployment content, applies to 2008 & 2008 R2. http://technet.microsoft.com/en-us/library/cc816638(WS.10).aspx