While there's much press coverage about Windows 7 hitting beta, the availability of the Windows Server 2008 R2 beta is also big news for me personally.
In particular this beta brings the first public look at the next iteration of Hyper-V. There's several very cool new features. Putting aside any marketing spin, two big features I'm glad to trumpet are:
Support for Hardware Address Translation: Basically, this means letting the CPU handle address translations across VMs, rather than the Hyper-V code doing page table remapping in software. If you have the right CPU (lookup "EPT" for Intel, "NPT" for AMD), you should see some nice performance gains in most scenarios. Most "current" AMDs have NPT already. Intel introduced EPT with the i7 (Nehalem).
Live Migration: The ability to move a VM running on one machine to another. The technology behind this is very impressive, and crazy complicated.
Of course, there are many more features than this. More details can be found in this Powerpoint presentation.