After a bit of poking around, here's a configure string which worked for me to build a 64-bit mplayer binary under Snow Leopard. Use at your own risk, as I've tested it on a grand total of two videos so far. I used the latest source available from git, following pigoz's instructions otherwise.
Code:
./configure --target=x86_64-Darwin --disable-x11 --disable-gl --disable-mencoder --disable-sdl --enable-apple-remote --enable-macosx-bundle --enable-macosx-finder --disable-liba52-internal --disable-mp3lib --disable-qtx --extra-cflags=-I/opt/local/include --extra-ldflags=-L/opt/local/lib
Make sure to add libmad and a52dec to the list of libraries you build with MacPorts. Once you run configure, edit config.h and change #define HAVE_EBX_AVAILABLE 1 to #define HAVE_EBX_AVAILABLE 0. Then make. (tip via
http://blog.bloople.net/read/mplayer-on-snow-leopard )