Since one hour ago, Qt-4.8.0 is in Gentoo portage tree. New major release so lots of new (or broken) stuff. The most important feature in this release is the integration of a new eselect module. This module will allow you to set your default graphics engine without the need to recompile Qt (x11-libs/qt-gui to be precise) from scratch. So, provided you have qt-gui-4.8.0 installed, you should be able to use the eselect module as follows:
hwoarang@mystical ~$ eselect qtgraphicssystem list
Available Qt Graphics Systems:  native  opengl  raster *
(note: if you have x11-libs/qt-openvg installed, one more option should be available)
Simply select your graphics system of preference, and then logout and login again.
hwoarang@mystical ~$ eselect qtgraphicssystem set 2 Setting opengl as your active Qt Graphics System... done Please logout for changes to take effect.
Thanks to Alex(wired) for the eselect module implementation.