Windows Server 2008 R2
RTM was with Windows 7 but it is now available for download on (at least TechNet Plus) and MSDN. This means that System Center Virtual Machine Manager 2008 R2 is not far away. This is probably the one System Center family member that has most synergy with OpsMgr. For more info about SCVMM R2 there is a webcast on 7th Sept.
Windows Server 2008 R2 is unique in that it is the first OS from Microsoft that is 64 bit only. That should trigger more applications to be 64 bit only. A 180 demo VHD for Hyper-V can be downloaded from here.