It looks like the unofficial git repository of mplayer, is now officially calling itself a fork and is actually making releases. The fork is called mplayer2, a decidedly unimaginative name, if you ask me. ^_^ This Phoronix post mentions the reasons for the sudden change in status.
mplayer2 is the only Linux video player that supports Matroska ordered chapters, an important feature for those of us who watch anime fansubs on Linux. However, what may of interest to more people is that mplayer2 builds against the multithreaded ffmpeg library by default for improved performance on modern systems.
I predict that if the mplayer2 makes frequent releases, it will quickly replace the original mplayer in most Linux distributions. mplayer releases stable versions so rarely that the prebuilt binaries in most Linux distributions are usually very dated. These old versions are missing features and bug fixes only available in the development version of mplayer. Since few people are capable of compiling from source, most Linux users end up with other media players such as vlc or xine.
Will mplayer2 succeed? Only time will tell.