Use StringBuilder instead of concatenating strings over and over. Also you should look at using string.charAt(index) instead of using substring for single chars:

import java.util.*;
import java.security.SecureRandom;

public class PassGen{

        private static final String VALID_PW_CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*()-_=+{}[]|:;<>?,./";
        private static final int DEFAULT_PASSWORD_LENGTH = 12;
        private static final Random RANDOM = new SecureRandom();


        // main class
        public static void main(String args[]) throws Exception {


                // Set password length
                int pwLength;
                if (args.length < 1)
                        pwLength = DEFAULT_PASSWORD_LENGTH;
                else
                        pwLength = Integer.parseInt(args[0]);

                StringBuilder pw = new StringBuilder();


                // generate password
                for (int i=0; i<pwLength; i++) {                        
                        int index = RANDOM.nextInt(VALID_PW_CHARS.length());
                        pw.append(VALID_PW_CHARS.charAt(index)));
                }

                System.out.println("pw = " + pw.toString());
        }
}

Also you are generating doubles and not restricting the index value. I did a mod of the length of the valid chars array to fix this problem.

If you use Apache Commons Lang, the easiest way is

RandomStringUtils.random(20, 0, 0, true, true, null, new SecureRandom());

You shouldn't use a random generator to generate unique keys. Nothing guarantees uniqueness. Why don't you use a UUID generator instead, or a database sequence?

And why do you want your keys to be properly distributed? The only important thing for a primary key is its uniqueness. And the seed doesn't have any impact on the distribution, AFAIK.