Network gaming has been around for a long time and is something that Steam has already got nailed - it may be mentioned in the anticipated Steam Box announce as it does differentiate that from the (common) Console/PC network segregation, but won't have an announce of it's own.
My guess is one of the next two announces will be a new game controller and the other will either be a Steam box console and/or a partnership with other vendors to produce a range of SteamOS compatible living room PCs.
Steam Big Picture mode already works with many existing controllers (The XBox 360 controller seems to be the most recommended), but they have previously indicated that they are very interested in advancing this area. We know their R&D has been churning away, but other than "motion is overrated" and "biometrics are interesting" we don't know any details yet.
As I see it these are the three pillars of a living room gaming PC - Software, hardware and a way to interact with it, but we'll find out more later in the week.