How can I override the @Html.LabelFor template?

You’d do this by creating your own HTML helper.

http://www.asp.net/mvc/tutorials/creating-custom-html-helpers-cs

You can view the code for LabelFor<> by downloading the source for ASP.Net MVC and modify that as a custom helper.

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Answer added by balexandre

public static class LabelExtensions
{
    public static MvcHtmlString LabelFor<TModel, TValue>(this HtmlHelper<TModel> html, Expression<Func<TModel, TValue>> expression, object htmlAttributes)
    {
        return LabelFor(html, expression, new RouteValueDictionary(htmlAttributes));
    }
    public static MvcHtmlString LabelFor<TModel, TValue>(this HtmlHelper<TModel> html, Expression<Func<TModel, TValue>> expression, IDictionary<string, object> htmlAttributes)
    {
        ModelMetadata metadata = ModelMetadata.FromLambdaExpression(expression, html.ViewData);
        string htmlFieldName = ExpressionHelper.GetExpressionText(expression);
        string labelText = metadata.DisplayName ?? metadata.PropertyName ?? htmlFieldName.Split('.').Last();
        if (String.IsNullOrEmpty(labelText))
        {
            return MvcHtmlString.Empty;
        }

        TagBuilder tag = new TagBuilder("label");
        tag.MergeAttributes(htmlAttributes);
        tag.Attributes.Add("for", html.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(htmlFieldName));

        TagBuilder span = new TagBuilder("span");
        span.SetInnerText(labelText);

        // assign <span> to <label> inner html
        tag.InnerHtml = span.ToString(TagRenderMode.Normal);

        return MvcHtmlString.Create(tag.ToString(TagRenderMode.Normal));
    }
}