Release Notes: libstdc++-v3 is the default C++ library for the GCC 3.0.x series of compilers. The previous branch (2.x) is obsolete and no longer maintained. There are compatibility issues between GCC compiler versions and libstdc++, requiring a custom library for each compiler version. Read http://gcc.gnu.org/libstdc++/status.html to make sure you get the right library for your compiler. Also, if you get strange errors while compiling a 3.x release of GCC (such as being told to upgrade to GCC 3.0), try updating libstdc++.
Release Notes: MT safe string, configure support for --enable-threads=posix as well as initial IO locking implementation, --enable-namespaces is on by default, configure and Makefile support for "drop-in" replacement to libstdc++-v2 completed, synched with CVS egcs libio, Cygwin native compiling supported, cross compiling and embedded targets (newlib) with multilibs support added, SGI's strstream implementation has been added, copyright on all sources assigned to the FSF, configure, build and install documentation has been added, support to enable long long has been added, more valarray improvements, extractors and inserters for std::complex have been added, extractors and inserters for void* have been fixed, autoconf macros are now in _GLIBCPP_ namespace, group checking for num_get implemented, and many bugs have been fixed.