I know this is kind of old and has been marked as answers with 147 up votes, but there is something else to consider.
You can have all the model errors, the property named and string.Empty keys alike, be shown in the ValidationSummary if you need to. There is an overload in the ValidationSummary that will do this.
// excludePropertyErrors:
// true to have the summary display model-level errors only, or false to have
// the summary display all errors.
public static MvcHtmlString ValidationSummary(this HtmlHelper htmlHelper, bool excludePropertyErrors);