Just a short code snippet that helped me recently:

public static string SerializeTableToString(
    DataTable table )
{
    if (table == null)
    {
        return null;
    }
    else
    {
        using (var sw = new StringWriter())
        using (var tw = new XmlTextWriter(sw))
        {
            // Must set name for serialization to succeed.
            table.TableName = @"MyTable";

            // --
            // http://bit.ly/a75DK7

            tw.Formatting = Formatting.Indented;

            tw.WriteStartDocument();
            tw.WriteStartElement(@"data");

            ((IXmlSerializable)table).WriteXml(tw);

            tw.WriteEndElement();
            tw.WriteEndDocument();

            // --

            tw.Flush();
            tw.Close();
            sw.Flush();

            return sw.ToString();
        }
    }
}

Hopefully it is useful for someone out there, too.