Has defining all of libcxx in another namespace ( libcxx?) and the 'inline namespace'ing it into std:: been considered? Obviously this would have to wait until there was compiler support.
This would have the advantage that any attempts to link code written in stlport/apachecxx/libstdc++ (where libstdc++ is the most serious concern) with code written using libcxx would fail to link. The obvious disadvantage is that people who look at demangled names would see 'libcxx'. Also if the compiler warnings mention the inlined namespace, that would also be confusing.