diff -r 5cf390d4fbe1 runtime/ftplugin/lisp.vim
--- a/runtime/ftplugin/lisp.vim	Thu Jan 17 18:34:06 2013 +0100
+++ b/runtime/ftplugin/lisp.vim	Fri Jan 18 17:37:25 2013 -0600
@@ -23,6 +23,7 @@
 " make comments behaviour like in c.vim
 " e.g. insertion of ;;; and ;; on normal "O" or "o" when staying in comment
 setl comments^=:;;;,:;;,sr:#\|,mb:\|,ex:\|#
+setl commentstring=;%s
 setl formatoptions+=croql
 
 let b:undo_ftplugin = "setlocal comments< define< formatoptions< iskeyword< lisp<"