Research firm Witsview is reporting that Apple is planning to release a 5K Thunderbolt Display and iMac in the fourth quarter of 2014. The display will reportedly feature the same 5120x2880 pixel resolution as the just-announced Dell UP2715K.
A 27" 5K monitor is not impossible
It is a rumor, but it is certainly not impossible. The interesting aspect is that the rumored panel with 5120x2880 pixels already exists and is coming to the Dell UP2715K, which will be released by years end. Similarly, Apple is rumored to release the 5K monitor and iMac by the end of the fourth quarter.
5120x2880 pixels would be exactly double (horizontally and vertically, but 4 times more pixels) of what Apple is offering in its current 27-inch iMac and Thunderbolt Display, which uses 2560x1440 pixels. This means that Apple can run it at 2X scaling just like it is doing with its Retina Macbooks, iPads and iPhones. So it appears plausible that Apple would promote such as display as the first Retina iMac or Thunderbolt Display.
However, Thunderbolt 2 (built on DisplayPort 1.2) might not be able to power the monitor, depending on the frame rate. Dell is said to use two DisplayPort 1.2 connections for its UP2715K monitor. A new DisplayPort 1.3 standard is currently being developed and Intel is also said to be working on a Thunderbolt 3 standard.
Previous rumors suggested that Apple was interested in launching a 4K monitor with 4096x2160 pixel resolution, which is supported by Thunderbolt 2.