It made a success, but now it's a bit out-to-date, as it was written for Hoary.
This stuff has started getting easier to configure since Breezy and now it's really very easy to have it work.
This HowTo will help you get WPA to work, no matter what wireless card you have. (This HowTo has been tested with an ipw2200).
So, let's start:

1) Open a terminal window and type:
Code:
wpa_passphrase your_ssid your_psk
Note: your_ssid is the name of your wireless network (a.k.a. SSID) and your_psk is the password you want to use to protect your network. (Look below for an example).

2) Now copy the psk string you got as output.

3) Type:
Code:
sudo gedit /etc/wpa_supplicant.conf
Then paste this as follow:
Code:
ctrl_interface=/var/run/wpa_supplicant
#ap_scan=2

network={
  ssid="your_ssid"
  scan_ssid=1
  proto=WPA RSN
  key_mgmt=WPA-PSK
  pairwise=CCMP TKIP
  group=CCMP TKIP
  psk=your_psk
}
Note: your_psk is the psk string you got from step 1.

Here is an example:
Code:
luca@laptop1:~$ wpa_passphrase mywlan thisisthepassword
network={
  ssid="mywlan"
  #psk="thisisthepassword"
  psk=b22ec921c254c73f99b31b76ff876692ecde36839a1f2d92150829e6afcb5515
}
The red string is what you have to paste into /etc/wpa_supplicant.conf as your_psk (without quotes obviously). So you'll have something like this:
Code:
ctrl_interface=/var/run/wpa_supplicant
#ap_scan=2

network={
  ssid="mywlan"
  scan_ssid=1
  proto=WPA RSN
  key_mgmt=WPA-PSK
  pairwise=CCMP TKIP
  group=CCMP TKIP
  psk=b22ec921c254c73f99b31b76ff876692ecde36839a1f2d92150829e6afcb5515
4) Save the file and close Gedit.

5) Now we have to make wpa_supplicant load when system boots, so go back to the terminal window and type:
Code:
sudo gedit /etc/network/interfaces
6) Add the following lines in the part regarding your wireless card, as in the example below:
Code:
pre-up wpa_supplicant -Bw -Dwext -ieth0 -c/etc/wpa_supplicant.conf
post-down killall -q wpa_supplicant
Note: "eth0" is your wireless device and "wext" is the driver; this is a kind of generic driver, so it should work with most wireless cards. If it doesn't, please try another driver, such as hostap, ndiswrapper, etc.
Here is an example:
Code:

iface eth0 inet static
address 192.168.1.15
netmask 255.255.255.0
wireless-essid my_essid
gateway 192.168.1.1
pre-up wpa_supplicant -Bw -Dwext -ieth0 -c/etc/wpa_supplicant.conf
post-down killall -q wpa_supplicant

7) Now run wpa_supplicant:
Code:
sudo wpa_supplicant -Bw -Dwext -i eth0 -c/etc/wpa_supplicant.conf
You should be online!



Troubleshooting:

You can run wpa_supplicant with -dd flag for a detailed debug output.

1) If you don't manage to connect to the AccessPoint, try to uncomment line 2 in /etc/wpa_supplicant.conf.
2) If that doesn't help, try change its value to 0 or 1.
3) If you get troubles while authenticating, try removing "RSN" and/or "CCMP" strings from /etc/wpa_supplicant.conf.