John, this comment (which is in the current version in CVS) is misleading, and IMHO the whole thing is redundant.
If *any* 3.4 libstdc++ header is included and _GLIBCXX_HAVE_GTHR_DEFAULT is defined then gthr-default.h will be included and therefore _REENTRANT will be defined, and therefore BOOST_HAS_THREADS will be defined anyway by boost/config/suffix.hpp (this is the case for GCC configured for MT support.)
If _GLIBCXX_HAVE_GTHR_DEFAULT is not defined then gthr-default.h will not be included, and _REENTRANT will not be defined, and BOOST_DISABLE_THREADS will be defined by boost/config/suffix.hpp (this is the case for GCC configured with no support for MT.)
I don't think that bit of preprocessor code changes anything (but I might be wrong again)
You're right, for some reason I thought we were dealing with gcc-3.3, but as you say it is indeed 3.4. In which case I'm lost as to why the config headers aren't doing the right thing anyway - including <utility> should be enough solve the issue as it does on linux :-( Thanks, John.