What??? Again???
Specific to 802.11i and Pre-Shared Key (used mainly for small networks)
Read here
Anyone with knowledge of the PSK can determine any PTK in the ESS through passive sniffing of the wireless network, listening for those all-important key exchange data frames. Also, if a weak passphrase is used, for example, a short passphrase, an offline dictionary attack can readily guess the PSK. Since the common usage will be a single PSK for the ESS, once this is learned by the attacker, the attacker is now a member of the ESS, and the whole ESS is compromised. The attacker can now read and forge any traffic in the ESS.
Pre-Shared Keying is provided in the standard to simplify deployments in small, low risk, networks. The risk of using PSKs against internal attacks is almost as bad as WEP. The risk of using passphrase based PSKs against external attacks is greater than using WEP. Thus the only value PSK has is if only truly random keys are used, or for deploy testing of basic WPA or 802.11i functions. PSK should ONLY be used if this is fully understood by the deployers.