Emerging Gstreamer 0.8.1 with USE="alsa -oss" should set the "/system/gstreamer/0.8/default/audiosink" gconf key to "alsasink" instead of "osssink" and the .../default/audiosrc key to "alsasrc" instead of "osssrc".
Alternately, installing gstreamer-dependant applications (such as rhythmbox) with the these USE values set should alter these gconf entries appropriately.
it's not a bad idea, but we really have no means to implement this at this point. A user can have alsa & oss & esd & artsd & nas all in USE, which to choose then? Only setting it when only one sink is in USE is confusing behaviour. Considering the unstable nature of gst-alsa & the lack of mixer for gst-esd , oss isn't a bad default choice at all for now.
This should at best only be done when installing gstreamer, but certainly not per-application. Applications themselves have no knowledge about src/sink, so we shouldn't imply that they have.
Closing as CANTFIX, because i think this cannot be done in a neat way currently.