Simple PHP Auth Class feedback
This part doesn't make sense to me:

private function crypt_pass($pass)
{
    if (CRYPT_BLOWFISH == 1)
    {
        $salt = SHA1('acssalty457');
        $blowfish_salt = "\$2a\$07\$".substr($salt, 0, CRYPT_SALT_LENGTH);
        return crypt($pass, $blowfish_salt);
    }
}


Functions that only return on condition are a bad idea. You should return something anyway, even if it's false. Now for the specific problem, what if CRYPT_BLOWFISH != 1? It's ok to want to use blowfist, but why cripple the library for everyone else? You could rewrite as:

private function crypt_pass($pass) {
    $salt = "acssalty457";

    if (CRYPT_BLOWFISH == 1) {
        $blowfish_salt = "\$2a\$07\$" . substr($salt, 0, CRYPT_SALT_LENGTH);
        return crypt($pass, $blowfish_salt);
    }

    return sha1($pass . $salt);        
}


Notice that I don't hash the salt? That's because security wise that's useless. You can ask around at Cryptography Stack Exchange for more. If you want to make the sha1 crypted pass a little bit harder you could do something weird like: 

return sha1(strrev($salt) . $pass . $salt);


I'm using sha1 as an example of a natively included function, you can use any hash algorithm you like.