Update (3/12): Going by name, one would assume the Xi3 was backed by Valve. Turns out, this isn't the case, as the developer has distanced themselves from the unofficial Steam Box. Valve's very own Doug Lombardi has said as much:
"Valve began some exploratory work with Xi3 last year, but currently has no involvement in any product of theirs."
So there you have it, the unofficial Steam Box is most definitely unofficial.
During CES, a number of hardware prototypes were shown off by Valve. All received the "Steam Box" moniker, and one of them, the Xi3 Piston, showed the most promise namely because Xi3 already has some products available. Now, that Piston can be added to the group as Xi3 has officially priced it at $999.99 and is now taking pre-orders.
This unofficial Steam Box is designed to sit under your TV and run Steam's Big Picture mode. Xi3's Piston is small enough to fit in your hand, so it should be just barely noticeable running games in your living room. It's $999.99 starting price includes an AMD A10 quad-core processor at 3.2GHz, 8GB of RAM, and a 128GB SSD, while a 256GB SSD bumps the price to $1,399.99 and a 512GB one to $1,749.99.
Anyone who pre-orders before March 18 can knock $100 off the final price and receive the Piston during the "2013 holiday season." If you're planning to wait until Valve gets its own Steam Box out, prototypes should be ready before long. In the meantime, there's the third-party offering from Xi3.