? hook_boot_cache_invalidate.patch
? sites/default/files
Index: includes/bootstrap.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/bootstrap.inc,v
retrieving revision 1.262
diff -u -p -r1.262 bootstrap.inc
--- includes/bootstrap.inc	30 Dec 2008 16:43:14 -0000	1.262
+++ includes/bootstrap.inc	31 Dec 2008 21:34:35 -0000
@@ -1146,8 +1146,10 @@ function _drupal_bootstrap($phase) {
         require_once DRUPAL_ROOT . '/includes/module.inc';
         module_invoke_all('boot');
       }
-      // If there is a cached page, display it.
-      if ($cache) {
+      // If there is a cached page, display it. However, the cache mode may be 
+      // altered by hook_boot, so refresh the value of $cache_mode first.
+      $cache_mode = variable_get('cache', CACHE_DISABLED);
+      if ($cache && $cache_mode) {
         drupal_page_cache_header($cache);
         // If the skipping of the bootstrap hooks is not enforced, call hook_exit.
         if ($cache_mode != CACHE_AGGRESSIVE) {