They call it a Pico-ATX which measure in at a whopping 10 cm x 7.2 cm!
The reference motherboard is very simple; you’ve got an Intel Atom CPU and a GeForce 9400M next to each other, a single SATA connector and a DDR3 SO-DIMM slot on the other side of the board. And this little thing is powerful enough to play HD video (8 – 25Mbps H.264) the CPU has been measured at 27% utilization while doing so. Ion also has full support for 8-channel LPCM over HDMI.
Built into the board is USB 2.0 ports, a dual-link DVI connector, an HDMI Connector, a SATA 3G connector, and a DDR3 SO-DIMM connection.
Nvidia also says that a GeForce 9400 paired with the Atom Processor will net 10 times the graphics and video transcoding, Fulll HD Video Decode and Display, and is half the size but same battery life. The reference board is Vista Premium and Windows 7 Ready.
For more interesting details check this out.