GStreamer has been proposed for inclusion in KDE-4.0. Currently, GStreamer is included as an optional component, and it's used by several KDE applications, including AmaroK, JuK, KMPlayer and Kaffeine.
Although not yet as complete as the GNOME integration bits, there are already some KDE integration specifics available. This list will probably grow as GStreamer starts to be used in KDE-4.0:
AmaroK contains a kiosrc element, which is a source element that integrates with the KDE VFS subsystem KIO.