Sonnet has announced it’s xMac Pro Server, to add the new MacPro to a Rack. Of course that is not all as it also adds PCI expansion through Thunderbolt 2, room for optical drives and ports. It is $1499, but adds 3 PCIe Ports to the MacPro including a Red Rocket (through Thunderbolt 2, so no graphics cards) with a 300W power supply, places (with additional kits) to install optical drives and additional hard drives.
A very cool idea, if a bit expensive.