Videos using the html5 <video> tag aren't played with QtWebKit on Linux with the Phonon Xine backend (or any of the other non-gstreamer backends).
I'm not sure the summary change reflects what this bug is about -- the bug occurs even with 4.6 and 4.5, when Phonon is used but it's used with the xine backend rather than the gstreamer one.
Ah, I see. Updating...
QtWebKit now uses the pure-gstreamer MediaPlayerPrivate implementation, which seems to work fine.