Right, but if the salt varies per user, then you end up doing a bruteforce on each user's password; it's no longer a precomputation attack. There are no "Rainbow tables" in this case.

However, if you find Hale's bcrypt page (http://codahale.com/how-to-safely-store-a-password/) convincing, and I do, salting really doesn't matter because with modern GPUs you can bruteforce a reasonably-sized alphanumeric password, if the hash algorithm is a general-purpose (read: fast) one.

The solution is not salt, the solution is to use a purposely slow hash function.