USE=rar enabled playing subtitles from rar files, but with the removal of unrarlib it now enables the unrarexec feature (which is great) -- unfortunately, this feature (even if enabled) is not usable in the default configuration. IMHO, USE=rar should set an RDEPEND on compatible rar packages we have and then add "unrarexec=/usr/bin/unrar" to the default mplayer.conf
Fixed in mplayer-20090226.28734.ebuild Thanks, Robert, did that exact thing.