Index: wp-settings.php
===================================================================
--- wp-settings.php	(revision 12600)
+++ wp-settings.php	(working copy)
@@ -615,10 +615,12 @@
 
 do_action('plugins_loaded');
 
-$default_constants = array( 'WP_POST_REVISIONS' => true );
-foreach ( $default_constants as $c => $v )
-	@define( $c, $v ); // will fail if the constant is already defined
-unset($default_constants, $c, $v);
+/**
+ * It is possible to define this in wp-config.php
+ * @since 2.6.0
+ */
+if ( !defined('WP_POST_REVISIONS') )
+	define('WP_POST_REVISIONS', true);
 
 // If already slashed, strip.
 if ( get_magic_quotes_gpc() ) {