Like a scramble method where "Hello, World!" becomes something like
"leol,H!o rlWd" ?

How about (off the top of my head):

       public static string Scramble(string input) {
           if(string.IsNullOrEmpty(input)) return input; // GIGO

           List<char> inputChars = new List<char>(input);
           char[] outputChars = new char[inputChars.Count];
           Random rand = new Random();
           // fill backwards to simplify Rand.Next() logic
           for (int i = inputChars.Count - 1; i >= 0; i--)
           {
               int index = rand.Next(i);
               outputChars[i] = inputChars[index];
               inputChars.RemoveAt(index);
           }
           return new string(outputChars);
       }

       public static void Main()
       {
           string foo = "The quick brown fox jumps over the lazy
dog";
           string bar = Scramble(foo);
       }

using System;

namespace E
{
    public class Program
    {
        private static Random rng = new Random();
        public static void Main(string[] args)
        {
            string s = "Hello world !";
            char[] c = s.ToCharArray();
            byte[] b = new byte[c.Length];
            rng.NextBytes(b);
            Array.Sort<byte, char>(b, c);
            string s2 = new string(c);
            Console.WriteLine(s2);
            Console.ReadKey();
        }
    }
}

It is not the most efficient method, but the code is simple.

Arne