As an alternative to the method I posted above for installing Steam on Debian, I built Ubuntu 12.04's libc 2.15 for Debian 7 AMD64 in a clean chroot and uploaded my unsigned binaries so you can install them instead of building it yourself (so long as you have a 64-bit Debian system). Just follow the instructions below to upgrade Debian 7's libc from 2.13 to 2.15 and install Steam.
# Download my libc tarball and the latest Steam beta package.
# Extract and install the libc packages.
tar -xJf eglibc_2.15-0ubuntu10.3_wheezy_deb.tar.lzma
sudo dpkg --force-overwrite -i libc6_2.15-*.deb libc6-dev_2.15-*.deb libc6-i386_2.15-*.deb libc6-dev-i386_2.15-*.deb libc-bin_2.15-*.deb libc-dev-bin_2.15-*.deb multiarch-support_2.15-*.deb nscd_2.15-*.deb
rm -rf eglibc_2.15-0ubuntu10.3_wheezy_deb
# Install the Steam package.
sudo apt-get install gdebi
sudo gdebi -n steam.deb