A view with exposed sort: can i turn off 'grouping field' for different sort criteria?
Ok, i think i've managed it with a Views hook. Took an hour's worth of tinkering. I plopped this into my template.php...

/** Find out what the sort criterion is.
    @return The sort criterion fieldname. IMPORTANT: this isn't sanitised!
*/
function MYTHEME_get_sort_by() {
    if (isset($_GET['sort_by']))
        return $_GET['sort_by'];    
}

/** Turn off Views grouping-field when sorting by title.
*/
function MYTHEME_views_pre_build(&amp;$view) {
    if ($view-&gt;name=='event_programme') {
        if (MYTHEME_get_sort_by()=='title')
            $view-&gt;display_handler-&gt;options['style_options']['grouping']='';    
    }
}