In order to enable wireless on the Lenovo ThinkPad X220 with Intel Centrino Advanced-N 6205 running Debian Wheezy, add non-free to /etc/apt/sources.list
# /etc/apt/sources.list
deb http://ftp.de.debian.org/debian/ squeeze main contrib non-free
deb-src http://ftp.de.debian.org/debian/ squeeze main contrib non-free
deb http://security.debian.org/ squeeze/updates main contrib non-free
deb-src http://security.debian.org/ squeeze/updates main contrib non-free
Then update the list of packages and install the needed firmware by issuing
sudo aptitude update; sudo aptitude install firmware-ralink firmware-iwlwifi wireless-tools
