I've seen info in various email-threads regarding thread safety when using xml archives. 
We had come across some assert conditions in multithreaded code, only to trace down that (BOOST_SPIRIT_THREADSAFE) was not defined during boost compilation.     

to fix, I modified the Jamfile.v2 for serialization:

lib boost_serialization
     : $(SOURCES).cpp
        /boost/thread//boost_thread                             # needed to add this as a dep b/c we need to link threads when we define BOOST_SPIRIT_THREADSAFE
    : <define>BOOST_SPIRIT_THREADSAFE            # needed to add this as a define
      <toolset>msvc:<cxxflags>/Gy
      <link>shared:<define>BOOST_SERIALIZATION_DYN_LINK=1
      <conditional>@include-spirit
    ;

I've also added BOOST_SPIRIT_THREADSAFE as a <define> in our code as well.

I hope this helps others...

Cheers,
Tim