// Bind event for ListItem PreRender, so we can add the Organization headings in the middle
                cblInvestigators.ItemPreRender += CheckBox_ItemPreRender;

Then I created the method CheckBox_ItemPreRender in the codebehind as well. This is a pretty sloppy version, but I’m handing the event the original list of data being bound, as well as the current index and list item being operated on. The HtmlTextWriter is what you’ll use to create output (kind of like Response.Write for classic ASP).

protected void CheckBox_ItemPreRender(System.Collections.IEnumerable data, int index, ListItem item, System.Web.UI.HtmlTextWriter writer)
        {
            // Get the item that was databound to the checkbox
            Model.Profile profile = data.Cast().ElementAt(index);

            // if this is the first item belonging to this org, show the header
            if (previousOrgId != profile.Org.Id)
                writer.Write("my header text that is different from the last row!");

            // store the orgId to compare against the next item
            previousOrgId = profile.Org.Id;
        }

Sorry if this does not format the code correctly in the comments, you may need to just cut and paste :)

“… On the page using the control, I hooked a method up to the Event that checked to see if the ‘Category’ was different than the last item”