Microsoft and ARM are working on a new version of the Windows RT operating system to support the new ARM chips under 64-bit RISC architecture.
As you know, Windows RT is a 32-bit versus Windows 8 (x86) version that has 32 and 64 bits. In the medium term is more than enough but after the announcement of the new 64-bit chips ARM (Cortex-A57 and Cortex-A53) for consumption, it will be interesting to have a version that can utilize their capabilities or extended memory management.
Also the 64bit native applications but unfortunately will be a large minority as with the x86 platform.
At present there is specific date of release for 64-bit ARM chips. A (Windows on ARM) that will be important in the coming years if ARM is exploding onto the PC market as it is doing in the area of mobility. This was all regarding Windows RT 64bit version, stay tuned for more updates.