I've been digging around on this and it appears the reason for the 60hz limitation on 2560 IPS panels is indeed due to the lack of bandwidth of DVI-D. The panel produced by LG is capable of 120hz - and has for some time - but the DVI interface does not allow for it within specs; it does not have enough bandwidth while displayport does..
Not quite correct. Dual Link DVI IS capable of the bandwidth for 120Hz at 2560x1440, however this bandwidth also needs to be supported by the GPU, its drivers, and the PCB of the monitor.
A couple niche IPS monitors exist that are capable of 120Hz at this resolution (Overlord Tempest X270OC and Yamakasi Catleap Extreme 2B). Both include the same PCB. This PCB does not have a '120Hz mode' out of the box. It needs to be overclocked.
The last couple gen of graphics cards are capable of supporting such overclocks. The current drivers from both AMD and NVIDIA are not. Both companies have imposed an arbitrary 330Mhz pixel clock limit in the drivers, which prevents you from reaching the needed bandwidth. So those of us that own these monitors are using customised drivers in which the pixel clock limit is unlocked.
That leaves the cable and DVI connection. It's true that the bandwidth needed to drive 120hz@2560x1440 tests the limits of DVI, but it is possible. It's true that the latest versions of DisplayPort offer even better bandwidth than DVI, and I understand that Thunderbolt could possibly do it to. A more eloquent and detailed explanation is available here:
http://120hz.net/showthread.php?1135-How-is-2560*1440-120hz-possible&p=17387&viewfull=1#post17387
What does this all mean? It probably IS possible for 2560x1440 monitors to offer 120Hz out of the box in the foreseeable future. However, a few things would need to happen before we'll see them from the big monitor manufacturers:
(1) A monitor PCB needs to be developed that comfortably supports the required bandwidth - I have no idea how feasible this is,
(2) Any monitor incorporating this fictitious PCB would need to have dual link DVI, or DisplayPort (or possibly Thunderbolt) among their inputs, and
(3) AMD and NVIDIA need to remove the pixel clock limits they currently impose in their drivers
How likely is this to happen in the coming few years? Who knows...
EDIT: kevinsbane beat me to it!