The AMD Open 3.0 motherboard is a 16" x 16.7" board designed to fit into 1U, 1.5U, 2U or 3U rack height servers.

ENGADGET: AMD unveils Open 3.0: an Opteron 6300 platform for the Open Compute Project

This system securely mounts a Mac mini inside a specially designed 1U enclosure, which also contains one PCIe 2.0 x4 slot, a 75W power supply plus a Thunderbolt daisy-chain port.

ENGADGET: Sonnet announces RackMac mini Xserver, makes Apple desktop IT life seem more legit

Taking the same 40 racks, if pushed to 90 percent capacity on average, and upgrading the existing server base over the next 2 years to 1U servers would support 1, 520 physical servers.

FORBES: The Case for the Infinite Data Center

The RackMac mini Xserver supports one half-length, x16 (x4 mode), PCIe 2.0 card to enable the use of professional video capture cards, 8Gb Fibre Channel cards, 10-Gigabit Ethernet cards, and RAID controller cards with a racked Mac mini all in 1U rack space.

ENGADGET: Sonnet announces RackMac mini Xserver, makes Apple desktop IT life seem more legit