Microsoft has made Windows Server 2008 R2 Service Pack 1 (and Windows 7 SP1) available on MSDN and TechNet earlier today. They are using the Akamai Download Manager for these, and I was able to download them very quickly with no problems.
They also have slipstream installs that incorporate SP1 into the full product that are available, which is a nice time saver. There is a single .iso file that includes the x86 and x64 versions of SP1 for Windows 7, and the x64 version of SP1 for Windows Server 2008 R2.
I burned the .iso file to a DVD, and then installed it on a fairly fast workstation (which has an Intel Core i7 930, 12GB of RAM, and a 128GB Crucial RealSSD C300 SSD that is plugged into a 6Gbps SATA III port). The Service Pack installation took 17 minutes to complete on this hardware. I really have to say that fast SSDs rock!
The most interesting new feature in this Service Pack (for Windows Server 2008 R2) is Dynamic Memory, which gives you greater control and flexibility about how Hyper-V allocates and manages memory among your guest VMs. This should let you run more VMs on a Hyper-V Host machine. You can see the differences in Figures 5 and 6, below.
Figure 1: Initial setup screen
Figure 2: Setup screen during installation
Figure 3: Final screen after reboot
Figure 4: Computer properties after SP1 installation
Figure 5: Old screen for Memory settings for Hyper-V VM
Figure 6: New screen for Memory settings for Hyper-V VM