After compiling mplayer-9999 the mplayer executable show the "UNKNOWN" version/revision. Compiling MPlayer locally sets normally current SVN revision number. Reproducible: Didn't try
The problem is caused by .svn being stripped by portage. One possible solution is proposed here: https://bugs.gentoo.org/show_bug.cgi?id=267124 See paragraph 10). You should add this to src_unpack.
Thanks for reply! In that case its all cleared things up. Its definitely not a bug then :)