If serialization depends upon BOOST_HAS_THREADS its totally unintentional on my part. I don't know where such a dependency would come from unless its shared_ptr which I only recently started to depend upon. I would like to see more information on this topic.
I just did a quick search for BOOST_HAS_THREADS: shared_count_132.hpp changes the ABI of sp_counted_base depending on whether it's defined or not. Whether that's an issue in practice I can't say, it depends on how that class is used. As for shared_ptr, it would indeed be useful to have more information, for example can I link a multithreaded lib that contains an API returning shared_ptr<foo> to a single threaded app? Looking at the source, it appears that for most platforms the answer is "yes": as long as shared_ptr recognises the platform and can use atomic increment/decrement operations, then those operations will always be used (and the internal ABI's remain unchanged as well). However it looks like there may be some platforms that rely on BOOST_HAS_THREADS / BOOST_HAS_PTHREADS, and I'm guessing that their ABI's will change. I'm not sure that this is the definitive answer though... John.