Updates
I'm no expert, but I would think that the graphics capabilities would need to be present at both the laptop and desktp level first before the monitor is updated. So, you may see the next Mac Pro able to provide 3840 x 2160 or 5120 x 2880 resolution through new video cards, but no "monitors" available yet to take advantage of that (I'm talking mainstream monitors, not the kind used for medical devices, etc.).
However, updates to the laptops in terms of resolution seem to be forthcoming and once they're updated, I imagine it wouldn't be too much longer before the 27" iMac and display are updated as well.
Just keep in mind that a major advantage such as super-high resolution is going to come out in their "Pro" models first.
It would be pretty sweet to have a 27" display or iMac with 5120 x 2880 resolution!!
Last thing, OS X Lion is rumored to have resolution-neutral capability built-in, so timing may also be a function of when the new OS is released as well. I'd bet that the Mac Pros would get the capability around the time the new OS is introduced or perhaps in the fall and then quickly followed by the MacBook Pro and Air lines along with the iMacs within a year (and probably 2 years or so for the Mac Mini).
Again, that's just a guess... retina-type displays may not become a reality at all in anything beyond an iPad...