Extension Methods to Reverse a String and StringBuilder Object

And even simpler, faster and better would be this:

public static string ReverseCA(string s) {
    char[] chars=s.ToCharArray();
    Array.Reverse(chars);
    return new string(chars);
}

Whatever the string's length, only three objects get created; and all characters get copied three times, no more, no less. For the same 80-char test string it is 14 times faster than John's original. Note: if you have to reverse a StringBuilder, the other alternative is probably the better one. :)