Qt Gstreamer is just a wrapper libary and the other two do a lot more. I don't know much about the history behind Phonon and Qt Multimedia so I won't go there. Phonon is probably bit more flexible than Qt Multimedia in sense that it support multiple backends on multiple platforms and Qt Multimedia only support GStreamer on Linux, whether such flexibility is needed is another question.
No announcement yet.
GStreamer 1.2 Feature Release Is Now Available