It seems to me that a 64-bit build would be a much desired version.
Has it been done or are there Qt 64-bit compiler issues blocking the way?
Yes SourcePole have created a 64 bit build. There should be an official 64bit build at some stage, there is funding and work happening, but no ETA at the moment.
To build a 64 bit version of QGIS you must also build all the dependencies as 64 bit. These include libspatialindex, gdal/ogr, etc.
A 64 bit build is now available, and the QGIS team are hoping people will start using/testing. http://faunaliagis.wordpress.com/2013/08/26/qgis-64bit-for-windows-is-ready-to-test/
There is a Google Summer of Code 2013 project on OSGeo4W ongoing which may cover that, see: http://trac.osgeo.org/osgeo4w/wiki/Reports
"Short description: Main goal for this project is develop functionalities in order to ask to the end users by an EULA license for proprietary libraries and building the majority of OSGeo components to x86-64 bits on Windows as well as improve the current OSGeo4W installer"