It was really quick to implement and works really nicely.

protected string ToTinyURLS(string txt)
{
    Regex regx = new Regex("http://([\\w+?\\.\\w+])+([a-zA-Z0-9\\~\\!\\@\\#\\$\\%\\^\\&\\*\\(\\)_\\-\\=\\+\\\\\\/\\?\\.\\:\\;\\'\\,]*)?", RegexOptions.IgnoreCase);

    MatchCollection mactches = regx.Matches(txt);

    foreach (Match match in mactches)
    {
        string tURL = MakeTinyUrl(match.Value);
        txt = txt.Replace(match.Value, tURL);
    }

    return txt;
}

public static string MakeTinyUrl(string Url)
{
    try
    {
        if (Url.Length <= 12)
        {
            return Url;
        }
        if (!Url.ToLower().StartsWith("http") && !Url.ToLower().StartsWith("ftp"))
        {
            Url = "http://" + Url;
        }
        var request = WebRequest.Create("http://tinyurl.com/api-create.php?url=" + Url);
        var res = request.GetResponse();
        string text;
        using (var reader = new StreamReader(res.GetResponseStream()))
        {
            text = reader.ReadToEnd();
        }
        return text;
    }
    catch (Exception)
    {
        return Url;
    }
}

Hope this can as useful to you as it was for me!