The controller was originally expected sometime this year, but evidently the powers that be don’t think it’s quite ready yet. The controller is currently undergoing live playtests “with everyone from industry professionals to die-hard gamers to casual gamers.” Valve says that the feedback has been helpful and is pushing back the release to make the controller even better.
A blog post tried to reassure those who are looking forward to getting their hands on the controller. It said, “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.”
Valve has a history of pushing things back until they’re ready; just look at the development cycle of Team Fortress 2…and Half-Life 3, of course.