Try this:
rmmod ath9k
modprobe ath9k nohwcrypt=1
Try to connect...
There's something wrong with the driver they've been packaging in Backtrack. Hopefully they fix this by 5r4 or 6r1
The crypt is not enabled by driver, so once loded need to remove module or driver and re-load it enabling the encryption (nohwcrypt=1) or create the file at /etc/modprobe.d/ath9k.conf and add the line:
options ath9k nohwcrypt=1
So every time the module is loaded, it will do it with the parameter enabled or do it manually as bahaeddine4 did..
Luck..