January 2013, but it turns out finalizing design, testing, and mass production of a VR headset takes a lot of time. You can get an idea of just how extensive the task is on Oculus' Kickstarter page, which depicts the process in detail.
As our friends at Engadget note, there are some signficant differences between the final dev kits and the prototypes first unveiled. There's now a 1280x800 7'' display as opposed to the original 5.6'' LCD, making the headset 30g heavier than it was before. Oculus says "the new display beats the old display in almost every key area including response time, switching time, contrast, and color quality," so an extra 30g is probably worth it.