QML is yet another new markup language for creating visual elements. It is not limited to that, but it is mainly used to build the graphical user interfaces for applications which in turn are done by using Qt library for C++. The idea in QML is the rapid development of any visual elements which in turn enables swift changes in case some usability research is done.
Most importantly, QML is written with a tool called Qt Creator, which is available for the three main operating systems, Linux, Mac and Windows.
QML (as in Qt Quick) should be shipped with Qt 4.7 once it becomes mature. This is expected to happen during this summer.