As far as I can tell, libavdevice requires media-video/ffmpeg-0.4.9_p20081014. This version, and everything depending on it, is currently hard masked. Unfortunately, this means >media-video/kdenlive-0.7 need to be added to the hard mask, as they require media-libs/mlt-0.3.0 with the ffmpeg USE flag.
it seems i have libavdevice in 0.4.9_p20080326
Ah, seems I shouldn't assume grep -l avdev is accurate :)
(in that case, this becomes a version dependency issue)
Reassigning to media-video, sorry.
on my system (amd64), mlt-0.3.0 does not build with ffmpeg-0.4.9_p20070616-r3 which is the latest "stable", but it builds without any problem with ffmpeg-0.4.9_p20080326
fixed, thanks