wpquestions.com, ID = 2781

<?php

// determine the topmost parent of a term
function get_term_top_most_parent($term_id, $taxonomy){

    // start from the current term
    $parent  = get_term_by( 'id', $term_id, $taxonomy);

    // climb up the hierarchy until we reach a term with parent = '0'
    while ($parent->parent != '0'){

        $term_id = $parent->parent;

        $parent  = get_term_by( 'id', $term_id, $taxonomy);

    }

    return $parent;

}

// so once you have this function you can just loop over the results returned by wp_get_object_terms
function hey_top_parents($taxonomy) {

    // get temrs for current post
    $terms = wp_get_object_terms( get_the_ID(), $taxonomy );

    // set vars
    $top_parent_terms = array();

    foreach ( $terms as $term ) {

        //get top level parent
        $top_parent = get_term_top_most_parent( $term->term_id, $taxonomy );

        //check if you have it in your array to only add it once
        if ( !in_array( $top_parent, $top_parent_terms ) ) {

            $top_parent_terms[] = $top_parent;

        }

    }

    // build output (the HTML is up to you)
    $r = '<p>Parent terms:</p><ul>';

    foreach ( $top_parent_terms as $term ) {

        $r .= '<li><a href="' . get_term_link( $term->slug, $taxonomy ) . '">' . $term->name . '</a></li>';

    }

    $r .= '</ul>';

    // return the results
    return $r;

}