How to Stop Debian Ignoring /etc/security/limits.conf

I was trying to increase the number of open files allowed for a user on a Debian 5 box. Changing /etc/security/limits.conf was being ignored until I added the following line to /etc/pam.d/common-session.

session required pam_limits.so

Logging out and in again after adding that line gave the new session the ulimit I was trying to configure.

Took quite a lot of googling to find this: http://www.zimbra.com/forums/administrators/16066-solved-too-many-open-files-error.html