Basic high-dpi support is now available for widgets and QML in Qt 4 and Qt 5.
Update (Aug 27th 2012):
Research on enabling HiDPI support on both Qt 4 and Qt 5 is underway. Short story is that anything that paints using CoreGraphics works well, but raster and opengl (as noted above) needs work. In practice this means that widgets on Qt 4 get HiDPI text, mac style, and QPainter vector output. Some additional work is needed to define how the points/pixels differentiation applies to Qt, especially with regards to images, pixmaps and icons.
Update (Dec 12th 2012):
Qt 4: Implementation work is underway in a separate repository: https://github.com/msorvig/qt4-hidpi
Qt 5: Basic high-dpi support has been merged to Qt 5.0. Further work is in this branch: http://qt.gitorious.org/qt/qtbase/commits/highdpi