Can't use apt-pinning (Hardy)

[Mumia W.]

I'm trying to use apt-pinning under Ubuntu Hardy i386 to prevent myself 
from accidentally installing a buggy library; however, this does not work:

$ cd /etc/apt
$ cat preferences
Package: libimlib2
Pin: version 1.4.0-1ubuntu1
Pin-Priority: -10

$
$ apt-cache policy libimlib2
libimlib2:
   Installed: (none)
   Candidate: 1.4.0-1ubuntu1
   Package pin: 1.4.0-1ubuntu1
   Version table:
      1.4.0-1ubuntu1 -10
         500 http://us.archive.ubuntu.com hardy/main Packages
$

Notice that the "-10" is in the wrong place. In Debian Etch's aptitude, 
"-10" would be located where "500" is, but in Ubuntu Hardy's aptitude, 
the default pin value of 500 is unchanged, and the "-10" shows up elsewhere.

Installs of libimlib2 are not prevented by aptitude:

$ aptitude -s install pypanel
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
Building tag database... Done
The following packages have been automatically kept back:
   linux-libc-dev
The following NEW packages will be automatically installed:
   libid3tag0 libimlib2 python-xlib
The following packages have been kept back:
   evolution evolution-common evolution-data-server
   evolution-data-server-common evolution-plugins libssl0.9.8 openssl
The following NEW packages will be installed:
   libid3tag0 libimlib2 pypanel python-xlib
0 packages upgraded, 4 newly installed, 0 to remove and 8 not upgraded.
Need to get 0B/411kB of archives. After unpacking 1700kB will be used.
Do you want to continue? [Y/n/?]
Would download/install/remove packages.
$

If I didn't know what I was doing, I could've accidentally installed 
libimlib2 (1.4.0-1ubuntu1) (exploitable). How can I get the pinning to work?