When will mingw 64 bit version available for Qt?
Originally one reason for choosing MinGW builds is that it has a uniform packaging for both 32 bit and 64 bit. And since Qt 5 is more mature than its introduction, it may be reasonable to support MinGW 64 bit in official build.
I know it still involves work to incorporate new compiler, but it should be nice to have if not that much effort is needed.
You can get a 64-bit MinGW package from http://sourceforge.net/projects/mingwbuilds/files/external-binary-packages/Qt-Builds/
See http://qt-project.org/forums/viewthread/24828/ (1 Oct 2013 and 5 Oct 2013) for an idea of why adding a new official build for Windows is not very practical.