Actually, make the change to hash opt-in, rather than opt-out. It's a bit late to introduce a breaking change.

[SVN r64007]

hash/doc/changes.qbk

@@ -108,10 +108,15 @@
 
 [h2 Boost 1.44.0]
 
-* Prevent implicit conversions when calling `hash_value`. If you find
-  that the new version breaks your code, you can enable the old
-  behaviour by defining `BOOST_HASH_ALLOW_IMPLICIT_CASTS` - although
-  I would recommend that you update your code to work with the new
-  version.
+* Add option to prevent implicit conversions when calling `hash_value` by
+  defining `BOOST_HASH_NO_IMPLICIT_CASTS`. When using `boost::hash`
+  for a type that does not have `hash_value` declared but does have
+  an implicit conversion to a type that does, it would use that
+  implicit conversion to hash it. Which can sometimes go very wrong,
+  e.g. using a conversion to bool and only hashing to 2 possible
+  values. Since fixing this is a breaking change and was only
+  approached quite late in the release cycle with little discussion
+  it's opt-in for now. This, or something like it, will become the
+  default in a future version.
 
 [endsect]

hash/test/Jamfile.v2

@@ -7,6 +7,7 @@ import testing ;
 
 project hash-tests
     : requirements
+        <define>BOOST_HASH_NO_IMPLICIT_CASTS
         <warnings>all
         <toolset>intel:<warnings>on
         <toolset>intel:<cxxflags>-strict-ansi

include/boost/functional/hash/hash.hpp

@@ -30,7 +30,7 @@
 
 namespace boost
 {
-#if !defined(BOOST_HASH_ALLOW_IMPLICIT_CASTS)
+#if defined(BOOST_HASH_NO_IMPLICIT_CASTS)
 
     // If you get a static assertion here, it's because hash_value
     // isn't declared for your type.