Use /var/run for storing dynamic mount information.
See also http://bugs.debian.org/551327

Index: libpam-mount-1.32/src/mtab.c
===================================================================
--- libpam-mount-1.32.orig/src/mtab.c	2009-11-17 05:06:17.000000000 +0100
+++ libpam-mount-1.32/src/mtab.c	2009-11-17 05:06:25.000000000 +0100
@@ -39,7 +39,7 @@
 typedef int (*scompare_t)(const char *, const char *);
 
 /* crypto mtab */
-static const char pmt_cmtab_file[] = "/etc/cmtab";
+static const char pmt_cmtab_file[] = "/var/run/cmtab";
 
 #if defined(__linux__)
 static const char pmt_smtab_file[] = "/etc/mtab";
