for people build their system completely from gcc-3.4, those binary packages that link against libstdc++.so.5 will fail. i.e. # ldd /opt/blackdown-jdk-1.4.1/jre/plugin/i386/mozilla/javaplugin_oji.so <snip> libstdc++.so.5 => /usr/lib/gcc-lib/i686-pc-linux-gnu/3.3.3/libstdc++.so.5 (0x4027a000) <snip> please take a look at bug #50596.
sys-libs/libstdc++-v3 serves this purpose