Xi3 has beaten everyone else to the punch by releasing the Piston for pre-order. This pint-sized gaming PC doesn't have the highest of performance parts; it features a quad-core AMD R464 processor and integrated graphics coupled with 8GB of RAM and a 128GB SSD. Larger SSDs are available for a price.
Upgrading to a 512GB SSD brings the price of the machine up by $750, a bit steep when you consider what SSDs are available for on Newegg. It's possible that Steam and Xi3 are pulling a Google in that they are releasing the very high-end systems before showing off more value-based propositions.
If you're interested in buying the Piston, Xi3 is offering a $100 discount for pre-orders that are placed before March 17. One of the few things this Steam Box has going for it is its small size and upgradability. But, are those enough to set this apart from the next generation PlayStation or Xbox? We'll have to wait and see.