120 Hz is not even imagined for delivery, because it would imply a large increase in bit rate for a small benefit, and the industry will not do that because bandwidth is expensive. In fact, what I see is the industry moving to 24P and allowing the TV itself to interpolate the other 36-96 frames, even up to interpolating to 960 frames locally. Once the installed universe of TVs is capable of this, which is only a few years away, program suppliers can switch to 24p and use the saved bandwidth for other, better purposes. Think of it as the grocery store selling you a 24-slice loaf of bread that automatically turns into a 120-slice loaf of bread once you place it in your magic breadbox at home.
There is actually a form of this sort of efficiency built into ATSC called film mode. Any original signal that was at one time 24p and later converted to 30i or 60p before broadcast, is stripped of the extra frames. IOW, only the original 24 fps second are delivered, and whatever the secondary format was is reconstituted inside your 1080p non-120 Hz-capable TV and displayed as 30i or 60p. That means that for film content from a 720p source, only 40% of what is displayed is ever delivered; the other 60% of what you actually are presented with is recreated locally, and perfectly accurately, directly inside the MPEG decoder. 120/240/480/960 is just an extension of the same technology, except that in that case the extra frames are each unique, rather than repeats of the original frames.
So 120 Hz is honestly just a display technology; a signal is transported in 24p, 30i, or 60p only, and the 120 only comes about during interpolation or cinema smoothing.
It is also still just ones and zeroes, so cabling that supports it is really just supporting higher bit rates; there is a metadata flag that tells the destination device what sort of pixel map and frame rate the transported signal will be parsed into, but other than that the signal is exactly the same, only possibly at a higher bit rate.
If the HDMI spec is elevated to handle "120 Hz", that is mostly a misdirect. It is conceivable that if you have a display with a 120 Hz light engine that also has a HDMI out that can be routed to a second display, that it might be able to be routed out as true 120 Hz, but it is doubtful that displays will ever really need to be capable of accepting a true 120 Hz signal. If they were capable of that, they would likely be capable of generating their own 120 Hz product locally, which precludes any possible need to be able to do that.
Edited by TomCat - 2/19/13 at 3:48pm