I had the same problem with my Keynux Impac (http://www.keynux.com/) with an Intel Pro Wireless 2200 BG. When booting the 4.0 DVD, I had unassociated essid. Iwlist could list all essid around.

I found the fix:
modprobe aes
modprobe arc4
modprobe crc32
modprobe michael_mic

and only then

iwconfig eth0 channel 10
iwconfig eth0 ESSID "my_essid"
iwconfig eth0 key "abcd1234..."