Ok, so no previous 1-series body has worked with an IR remote...it seems that the 1D X does.
That little transparent-looking window is in exactly the same spot as the one on my 7D, which is the IR remote sensor. According to Imaging Resource: "Physically, the Canon 1D X doesn't veer much from its predecessors, but with a few notable additions. One prominent addition is the infrared port on the front of the grip, a feature only recently seen moving up the line from the Rebels, and now it's made it to the top. If you wanted infrared on a pro or semi-pro camera, you had to pay for an expensive receiver and remote combo, but now it's built into the EOS-1D X. We have yet to learn whether it requires a special remote."