Windows 8 Release Preview is here.
x86 and x64 ISO images available for download.

Windows Server 2012 Release Candidate is here.
ISO image (product only ships as x64) and pre-installed VHD available for download.

Both are based on build 8400 of common source code.

The last set of scale/support figures for Hyper-V on Server 2012 are very interesting reading:
- Up to 320 virtual processors per host (increased from 64)
- Up to 64 virtual processors per VM (increased from 4)
- Up to 4TB memory per host (increased from 1TB)
- Up to 64TB per virtual disk (new VHDX format, VHD limitation is 2TB)
- Up to 64 nodes in a cluster (increased from 16)
- Up to 4,000 VMs in a cluster (increased from 1,000)

As with the previous Consumer Preview & Beta releases, these will be functional until January 2013.