Stephen Berman <Stephen.Berman at gmx.net>:

(recentf-save-file): Add a custom :set function.

lisp/recentf.el

@@ -72,7 +72,14 @@ See the command `recentf-save-list'."
 (defcustom recentf-save-file "~/.recentf"
   "*File to save the recent list into."
   :group 'recentf
-  :type 'file)
+  :type 'file
+  :initialize 'custom-initialize-default
+  :set (lambda (symbol value)
+         (let ((oldvalue (eval symbol)))
+           (custom-set-default symbol value)
+           (and (not (equal value oldvalue))
+                recentf-mode
+                (recentf-load-list)))))
 
 (defcustom recentf-save-file-modes 384 ;; 0600
   "Mode bits of recentf save file, as an integer, or nil.