Since KDE 4.9.0, libkcddb has been updated to use libmusicbrainz-5, so the dependency media-libs/musicbrainz:3 in libkcddb-4.9.0.ebuild should be changed to media-libs/musicbrainz:5
Thank you for the report. This is fixed in CVS now. Please sync in some hours to get changes. + 06 Aug 2012; Johannes Huber <johu@gentoo.org> libkcddb-4.9.0.ebuild: + Depend on kde-base/musicbrainz:5 by Richard Lärkäng <larkang@gmail.com> wrt + bug #430076.