Recently I discovered on Debian Wheezy that they are introducing /tmp automatically mounted as tmpfs. On current Debian Wheezy, this has been changed.

Check the old advisory

# man 5 rcS
...
NOTE
       The EDITMOTD, RAMRUN and UTC variables are no longer used.  The RAMLOCK, RAMSHM and RAMTMP variables have been moved to /etc/default/tmpfs; RAMSHM and RAMTMP settings in rcS are used (if set) for
       backward compatibility, but will be overridden by settings enabled in /etc/default/tmpfs.  See tmpfs(5) for further details.  The settings are not automatically  migrated  to  /etc/default/tmpfs.
       Please  update  /etc/default/tmpfs  appropriately.   The  UTC  setting  is  replaced  by the UTC or LOCAL setting in /etc/adjtime, and should have been migrated automatically.  See hwclock(5) and
       hwclock(8) for further details on configuring the system clock.

Which is now handled within tmpfs(5).

# man 5 tmpfs
...
       RAMTMP Mount /tmp as a tmpfs.  Defaults to no; set to yes to enable (/tmp will be part of the root filesystem if disabled).  /tmp may also be configured to be  a  separate  mount  in  /etc/fstab,
              which will override the RAMTMP setting.

So for gods sake, this is disabled by default now (# mount will tell you). If you require this to be enabled, set this accordingly in /etc/default/tmpfs – which is pretty nice, thanks Debian!

# vim /etc/default/tmpfs
...
# mount /tmp as a tmpfs.  Defaults to no; set to yes to enable (/tmp
# will be part of the root filesystem if disabled).  /tmp may also be
# configured to be a separate mount in /etc/fstab.
#RAMTMP=no

# Size limits.  Please see tmpfs(5) for details on how to configure
# tmpfs size limits.
#TMPFS_SIZE=20%VM
#RUN_SIZE=10%
#LOCK_SIZE=5242880 # 5MiB
#SHM_SIZE=
#TMP_SIZE=