Read 426. Rainbow Crack is a brute force attack, but the brute force is calculated in advance by a bunch of computers adding to the lookup table. As of now it only does 14 characters, and could therefore not handle a salted password of any decent length if you count the salt simply as part of the password. I guess there is a small probability that it could run across a short, salted password that is in its lookup table. Any Linux password of greater than six characters (assuming an eight-character salt) would be outside its range, meaning it couldn't crack the average 8-character long password.
In any case, it's success rate on Linux would be nowhere near the 99.9% success rate it has on Windows passwords up to 14 characters in length.
wasn't it you that said Linux was using a 4 character salt? It may have been someone else.
But what size salt does the typical Linux build use?