More or less some notes to myself again:

(Assuming, of course, that the wlan interface is wlan0.)

The magic command for generating the required PSK for WPA/WPA2 is wpa_passphrase.

janne@pingu:~$ wpa_passphrase ExampleSSID DamnLongPassword
network={
  ssid="ExampleSSID"
  #psk="DamnLongPassword"
  psk=9981ece75e0eb563acbc783b1ce4250e7383d9ce7f71caa0d8406c0d1495f4bb
}

When that’s done add the information to /etc/network/interfaces:

auto wlan0
iface wlan0 inet dhcp
wpa-ssid "ExampleSSID"
wpa-psk 9981ece75e0eb563acbc783b1ce4250e7383d9ce7f71caa0d8406c0d1495f4bb

…and finally you might want to remove network-manager to avoid conflicts.

Now this combined with automatic login lets me reboot my laptop at home without having to a) login and b) connect to the wireless.