Show ValidationSummary MVC3 as “alert-error” Bootstrap

edited again

I misunderstood your question at first. I think the following is what you want:

@if (ViewData.ModelState[""] != null && ViewData.ModelState[""].Errors.Count > 0)
{ 
    <div class="alert alert-error">
        <button type="button" class="close" data-dismiss="alert">×</button>
        @Html.ValidationSummary(true, "Errors: ")
    </div>
}