The "ffmpeg-mt" branch was created back in 2008 as a Google Summer of Code project to provide support for multi-threaded decoding. This was considered an experimental branch but with time has become usable and widely used for speeding FFmpeg on multi-core PCs.
The Austrian Michael Niedermayer has merged ffmpeg-mt to the main FFmpeg trunk code-base, as said in this message. It's also passing all of their FATE regression tests now. It was also announced on the FFmpeg home-page.
Today FFmpeg-mt, the multithreaded decoding branch, has been merged into FFmpeg. This has been a long awaited merge, and we would like to thank Alexander Strange for his patience and hard work.
Testing is appreciated and if you find any bugs please report them to our bug tracker.
The FFmpeg team is also tagging the 0.6.2 release right now (libav tagged their 0.6.2 release last week with the same changes and also updating their information to reflect the new project name). The FFmpeg 0.6.2 release does not provide this multi-threading support but that would to be expected for FFmpeg 0.7.