I've tried running some of the headers through the preprocessor, <iterator> looks like the smallest (but still much larger than <iosfwd> & <utility>). Although, <string> might be a better choice, as it's more likely to be included elsewhere.
Sigh, sadly no can do, they all end including one of the TR1 "poisoned" headers (<memory> etc) eventually, and you end up with cyclic dependencies, the problem is basically: Boost.Config -> includes <string> (for example) .... includes <memory> finds the TR1 version, which... includes <boost/shared.hpp> which requires Boost.Config to have been fully parsed already, but it's not, we're still in the middle of it! There are other cyclic issues that are *much* worse than this, and I have a nasty feeling I haven't found all of them yet. It basically sucks, and I've been tearing my hair out over this, because I also have a nasty suspicion that the TR1 headers will crash and burn if some of the Boost headers are included first. Clearly that's not acceptable. I think I have a solution that can be applied to Boost.Config and will let us go back to using <utility> which has proved itself remarkably reliable, but it's intrusive and can't really be applied to Boost as a whole. So... I think I'm going with plan Z (the other 25 having been exhaused!), and I'll revert the recent change, and go back to the drawing board... John.