Other possibilities: After the sensor, I think the headphones or one of the cables are the next most likely part. I believe the cables are also custom made (split HDMI/USB), they would not be packaged until late in the process, and the rift obviously cannot be used without them.
Nobody thinks it was the lenses? I had seen that mentioned previously as a rumor. That would be worrisome since you have to pretty much disassemble the entire headset to fix that.
How disgustingly hilarious would it be if the xbone controller - the one thing that none of us even want - is the "missing component" that's causing these delays.