To have a RTSP client, you need to process the RTSP protocol one way or another.
Live555 is one way to do that, it is just a C++ library that can be linked with other applications, including Qt. It is certainly possible to link Live555 with Qt.
Another way would be to write your own RTSP client based off the RFC spec .
The last option would be to just use the Phonon framework in Qt. http://doc.trolltech.com/4.6/phonon-overview.html (provided your Phonon backend will support RTSP). This is the easiest way as Qt and the system handle all the backend decoding of the media, integrates seamlessly with Qt and does not require extra libraries to be linked in with your app.