It seems that there is a minor problem with Debian 3.1r0 CD/DVD images. Here is a news about it.

Note: 3.1r0 CD image problem A bug has been discovered in the 3.1r0 CD/DVD images: new installs from these images will have a commented-out entry in /etc/apt/sources.list for http://security.debian.org/ testing/updates rather than an active entry for http://security.debian.org/ stable/updates, and thus will not get security updates by default. This was due to incorrect Release files on the images.

If you have already installed a system using a 3.1r0 CD/DVD image, you do not need to reinstall. Instead, simply edit /etc/apt/sources.list, look for any lines mentioning security.debian.org, change "testing" to "stable", and remove "# " from the start of the line.

If you installed other than from a CD or DVD (for example, netboot, or booting from floppy and installing the base system from the network), you are not affected by this bug.

New 3.1r0a images will be available shortly to correct this flaw. We apologise for the inconvenience.