Guest OS support additions (integration components):
Windows Server 2003 (x86) SMP, Windows Vista SP1 (x86) and Windows XP SP3 (x86)
1. Performance fixes for scalability and throughput for workloads.
2. VMs using pass-through disks will see a substantial performance improvement.
3. Installing Hyper-V VMs via network (RIS/PXE/WDS) is greatly improved.
You will want to read over KB 949219 before blindly updating your Beta VMs to RC VMs. I personally have run into some issues trying to import my older VMs into the new build.
The main Hyper-V website, which also is not online quite yet, is going to be located here.
From the announcement here, you can find more information on the new build.
Update: You can download the x64 build here and x86 build here. Release notes here.