Thinkpad X60s, WiFi and Debian

You will need following packages:

• wireless-tools
• wpasupplicant
• firmware-iwlwifi
• >=linux-image-2.6.23

After installing the packages create wpa_supplicant.conf (change 'your_ssid' to your router SSID and 'your_password' to your real passphrase):
# wpa_passphrase your_ssid your_password > /etc/wpa_supplicant/wpa_supplicant.conf

Open /etc/wpa_supplicant/wpa_supplicant and make sure it looks like this:

ctrl_interface=/var/run/wpa_supplicant

eapol_version=1
ap_scan=1
fast_reauth=1

# Only WPA-PSK is used. Any valid cipher combination is accepted.
network={
  ssid="your_ssid"
  proto=WPA
  key_mgmt=WPA-PSK
  pairwise=CCMP TKIP
  group=CCMP TKIP
  priority=2
  #psk="your_password"
  psk=6d5324610d3627ab4f97b80cf22b742996d82c022b283a874e88d083a299734c
}

Open and add to /etc/network/interfaces following text:

# Wireless
allow-hotplug wlan0
iface wlan0 inet manual
       wpa-drive wexp
       wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp

Run this command and you're done:
# ifconfig wlan0 up

More information can be found on Debian Wiki.