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
QTabBar may not be rendered correctly if tabs contain icons on High Dpi. The QTabBar height is twice the correct size and text and icon are rendered outside tab.
(screen shot attached) and a little patch seems to solve this problem is attached for consideration.