P.S. we already document how to link applications using two incompatible versions of libstdc++: http://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html#abi.testing.multi
(whether you can actually make that work in practice is another matter!) The situation is similar for C++98 and C++11 code, if the incompatible standard library types aren't used in the library interfaces then they should be able to coexist and not interfere. If in a future release we use namespace and/or symbol versioning to keep the c++98 and c++11 code distinct then they could coexist in the same library and won't interfere at all.
