This updated version of youtube-dl includes a new dependency, media-video/rtmpdump, for which I have just submitted an ebuild.
Created attachment 215346 [details] updated ebuild for youtube-dl-2010.01.05
Current version is youtube-dl-2010.04.04. Again, note that all versions youtube-dl-2010.01.05 and greater "require" rtmpdump (it will work fine without rtmpdump as long as it doesn't run into a video that needs rtmpdump to download), which is not in portage.
Latest version in, rtmpdump support will follow when I get it in portage.