If you were hoping to purchase a Valve-stamped device then you will be disappointed.
Valve, as it becomes increasingly clear, does not want to limit SteamOS to specific hardware. With the tag line, "Finally, a multiple choice answer", Valve wants consumers to purchase from OEMs or create the devices themselves.
Valve will make 300 of their own boxes and deliver them to selected beta testers, for free, after an "eligibility quest" ending October 25th. No specifications have been announced for these devices except that they are high performance, upgradable, and open. Even if you do not get one of these boxes, completing the quest will earn you a Steam badge so, that is something, right?
The most important announcement, hidden in the FAQ, is that game streaming will be available during the Beta test. I could assume, from this, that it will be available at launch. This allows users to access "the 3,000 games on Steam" whether running natively or networked to your gaming computer. Also in the FAQ, SteamOS will have mouse and keyboard support although it clearly is designed for gamepad input, too.
The longer this goes, the more correct I feel about Valve picking up the slack left behind by Microsoft. These boxes look at consoles from the model of "Media Center Extenders" except with Steam and other streaming partners being the Media Center server instead of actual Windows Media Center. Sure, I expect them to be more powerful than Roku boxes and many even more powerful than the Xbox One and PS4, but they are looking to follow that market segment.
I do not see these devices even trying to compete with PC market share.
You can purchase your own Steam Machine from a number of OEMs in 2014. The beta contest closes October 25th and those devices will be shipped between now at the end of the year. For details on the "eligibility quest", check out Steam's page.
Make sure to come back on Friday for the last of three announcements. Also, if you're around in 45 minutes (after publish), check out AMD's Hawaii GPU announcement live stream.