Longer: it can't run at any resolution: Target Display Mode has been dropped from the Spec' sheet. It was listed under "Video and Camera Support" for the old models: http://www.apple.com/imac/specs/.
This makes perfect sense given the bandwidth limitations of Thunderbolt compared with the requirements of a '5k' display.
There are lots of very disappointed Mac pro users out there who firstly didn't get a tb2 display and secondly they can't use one of these in target using tb2!