Harmy said:

but all the post processing is still being done at 2K - even for films such as The Hobbit, which was shot 5K but the finished film simply doesn't exist at a resolution higher than 2K, because a vast majority of digital projectors in cinemas all over the world are 2K or less anyway.

It needs to be noted though that the Hobbit WAS planned to be finished at 4K (and it could have been sized down for the needs of digital projection without problems after that), but since 4K requires much longer render times and general fidelity, it was actually time constratins (and probably budget) that forced WETA to finish it at 2K.

Of course, having the movie at 3D and 48fps doesn't really make things any easier projection wise.