HDPLEX has just unveiled their DC-ATX 800W, which is a nano-ATX form-factor power converter. About the size of a power brick this device takes in 6-pin PCIe, 8-pin PCIe, and 8-pin EPS connectors from your main rig and outputs 800W of power through a 24-pin ATX, and three downstream PCIe/EPS connectors. The unit itself will handle the DC-to-DC switching of 12V to 5V and 3.3V voltage domains.
Something like this would be ideal if you have a higher wattage power supply with vacant EPS and PCIe connectors and want to power a second smaller mini-ITX system within your rig. The unit makes use of some pretty high-grade components like Sanyo Japan-made solid-state capacitors, Infineon-made MOSFETs, Würth Elektronik inductors, and controller chips supplied by National Semiconductor and MPS. The DC-ATX 800W will be available very soon for $190.