libav-10 has been unmasked in Gentoo. The result is that systems with media-plugins/gst-plugins-ffmpeg are now broken, since libav 10 is not supported. Please place a block on >=libav-10 in gst-plugins-ffmpeg.
Or, alternatively, please add a USE flag to build gst-plugins-ffmpeg against the version of ffmpeg (or is it libav?) that comes with it. The current situation is a PITA; I can't upgrade libav just because of this.
*** This bug has been marked as a duplicate of bug 509326 ***