Mix & Match of KDE3 and KDE4 application is possible without problems. We've been doing that for a long time, especially as it took applications some time to port from Qt3 / KDE3 to Qt4 / KDE4.
Ironically, what we heard especially from the GStreamer team were complaints about the introduction of Phonon, which provides an abstracted multimedia API. It has backends for GStreamer, VLC and a few other things. The GStreamer developer's arguments were that Phonon is entirely unnecessary and that they would provide an API with guaranteed source and binary compatibility. If we'd listened to them, we'd be stuck with GStreamer as of two API/ABI versions ago. I've just talked to the developer who's working on the phonon-gstreamer backend, and he says the necessary changes have already hit master. We'll be able to provide the latest GStreamer to our users without them having to install new versions of their applications. It's a little tempting to now say "told you so". With today's knowledge and experience, I'd still call the collaboration between the phonon developers and the gstreamer team a success story, even if it wasn't up to a perfect start.
I'm happy about the 1.0 release. It's a sign of confidence and maturity, and a stable and mature multimedia framework is most welcome.
Congrats to the GStreamer team, have some cake. :)