baselibs doesn't include expat 1.x which some i386 binary applications depend on, like media-video/mplayer-bin and app-text/acroread-8.1.1
/opt/mplayer-bin/bin/mplayer-bin: error while loading shared libraries: libexpat.so.0: cannot open shared object file: No such file or directory
/opt/Adobe/Reader8/Reader/intellinux/bin/acroread: error while loading shared libraries: libexpat.so.0: cannot open shared object file: No such file or directory
baselibs does not have a libexpat.so.0. mplayer-bin links to libexpat.so.1 and you should be using /opt/bin/acroread instead. I suspect you have some outdated software on your system. Here's what I'm running:
[ebuild R ] app-text/acroread-7.0.9-r1
[ebuild R ] media-video/mplayer-bin-1.0_rc1-r3
False alarm--I had updated a few of the packages (like baselibs) but didn't notice that gtklibs had not been updated, and the old pango wanted expat-2. Acrobat8 and mplayer-bin-1.0_rc1-r3 work now.