Worse still, after spending another 2 hours looking into it last night, I found that it is incredibly hard to tell what cables are what. Sites say that legally speaking, the HDMI group doesn't let cable vendors label their cables as "1.4" or "2.0". They are only allowed to say "High Speed HDMI", which is near useless in telling you anything.
It turns out that it's not just rez but refresh that has to be factored into your cable purchase. I guess that's a holdover from HDMI's TV focus. TV's are fine with 30fps. Most computer people need 60fps.
For instance, you *need* HDMI 2.0 to do 4k@60. But they can't label their cable as "2.0", so WTF are you supposed to do?
I talked with my reseller rep at Startech and they said none of their HDMI cables are 2.0 even though they loudly exclaim "ultra hd!" and "4k!". He said they are all HDMI 1.4 or 4k@30. Luckily 1.4 will also do my required 2560x1440@60, so in my case 1.4 is good enough. However, if I was super bleeding edge I'd be getting a 4k monitor and then be stuck having to find a 2.0 cable amongst these wacky labeling rules.
C2G's HDMI cables claim to do 4k@60, so there's a safe option... unless they are lying!
P.S. Michael is almost certainly right when he said at the meeting that you have to manually add the 2560x1440 rez into xrandr once you have the correct components in place.
P.P.S. It looks like 2560x1440 is doable with HDMI, just with so many caveats most people can't get it working and switch to DP. I've picked out a cable and a new inexpensive VC that should allow me to get this working. (And vdpau, which I've wanted for a while...) I will report back in a few weeks.