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.
These new 3.1_r0a images correct this flaw. We apologise for the inconvenience.