Valve has just posted an update to the Steam Universe community. Long story short, they're back to experimenting with wireless controllers and are conducting live play tests with these new controllers. These play tests are generating a lot of useful feedback, but now with the time to incorporate these improvements, "we're now looking at a release window of 2015, not 2014." Though it's a bit unclear whether this will hold back a majority (all?) of the Steam Machines or whether just the top-tier, best units are now a year away.
The update also says, "Obviously we're just as eager as you are to get a Steam Machine in your hands. But our number one priority is making sure that when you do, you'll be getting the best gaming experience possible. We hope you'll be patient with us while we get there. Until then, we’ll continue to post updates as we have more stories to share."