Then, removing the #if !defined( BOOST_NO_CXX11_NULLPTR ) guard from the constructor of shared_ptr that accepts a boost::detail::sp_nullptr_t does indeed seem to work.

namespace boost {
   namespace detail {
      struct sp_nullptr_t {};
   }
   const detail::sp_nullptr_t nullptr = detail::sp_nullptr_t();
}

#include <boost/smart_ptr/shared_ptr.hpp>

int f(boost::shared_ptr<int> i) { return i ? *i : 0; }

int main()
{
   f(boost::nullptr);
}

...there's probably some reason I'm not thinking of.