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.