Index: modules/block/block.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/block/block.module,v
retrieving revision 1.427
diff -u -p -r1.427 block.module
--- modules/block/block.module	31 Jul 2010 12:54:58 -0000	1.427
+++ modules/block/block.module	10 Sep 2010 17:49:17 -0000
@@ -317,6 +317,10 @@ function block_get_blocks_by_region($reg
   $build = array();
   if ($list = block_list($region)) {
     $build = _block_get_renderable_array($list);
+
+    // Allow modules to alter the region, either via
+    // hook_block_region_REGION_alter() or hook_block_region_alter().
+    drupal_alter(array("block_region_{$region}", 'block_region'), $build, $list, $region);
   }
   return $build;
 }