I don't understand why you think limiting the length of plaintext passwords to say, 12 to 16 characters will mitigate or eliminate the threat. Surely all that would do is reduce the search space that the attacker has to try to find a matching MD5 hash, making it even easier and quicker to crack the system. That's unless you think that the attacker won't know that you are limiting password length, in which case, aren't you relying on "security through obscurity"? As we all should know, security through obscurity gives a false sense of security, rather than actual security.