Eric Lippert's challenge “comma-quibbling”, best answer?

Question

I wanted to bring this challenge to the attention of the stackoverflow community. The original problem and answers are here. BTW, if you did not follow it before, you should

Answer 1

I don't think that using a good old array is a restriction. Here is my version using an array and an extension method:

public static string CommaQuibbling(IEnumerable list)
{
    string[] array = list.ToArray();

    if (array.Length == 0) return string.Empty.PutCurlyBraces();
    if (array.Length == 1) return array[0].PutCurlyBraces();

    string allExceptLast = string.Join(", ", array, 0, array.Length - 1);
    string theLast = array[array.Length - 1];

    return string.Format("{0} and {1}", allExceptLast, theLast)
                 .PutCurlyBraces();
}

public static string PutCurlyBraces(this string str)
{
    return "{" + str + "}";
}

I am using an array because of the string.Join method and because if the possibility of accessing the last element via an index. The extension method is here because of DRY.

I think that the performance penalities come from the list.ToArray() and string.Join calls, but all in one I hope that piece of code is pleasent to read and maintain.