diff -u -r -p boost/thread/win32/once.hpp boost.optimized/thread/win32/once.hpp
--- boost/thread/win32/once.hpp	2009-12-17 20:51:58.000000000 +0100
+++ boost.optimized/thread/win32/once.hpp	2010-11-17 15:57:41.000000000 +0100
@@ -112,10 +112,11 @@ namespace boost
     void call_once(once_flag& flag,Function f)
     {
         // Try for a quick win: if the procedure has already been called
-        // just skip through:
+        // just skip through. Also try to do a non-interlocked read first which
+        // should only see the old(=not initialized yet) value in extreme rare cases.
         long const function_complete_flag_value=0xc15730e2;
 
-        if(::boost::detail::interlocked_read_acquire(&flag)!=function_complete_flag_value)
+        if(flag!=function_complete_flag_value && ::boost::detail::interlocked_read_acquire(&flag)!=function_complete_flag_value)
         {
             void* const mutex_handle(::boost::detail::create_once_mutex(&flag));
             BOOST_ASSERT(mutex_handle);