rviz_qt is a port of rviz to the Qt GUI library. It is a temporary package in a temporary stack, to exist only until "unstable" (leading to Fuerte) is stable enough to build it and be useful. After that, rviz_qt will be copied over the existing rviz package to become rviz 1.7.

For a list of changes between rviz and rviz_qt, see visualization_experimental/ChangeList.