Display linked categories and the posts within those categories.

functions.php

<?php

remove_action( 'genesis_loop', 'genesis_do_loop' );
add_action( 'genesis_loop', 'gmj_category_listings' );
/**
 * Display linked categories and the posts within those categories.
 * 
 * @author Gary Jones
 * @link   https://gist.github.com/1747188
 */
function gmj_category_listings() {

	// Get array of category objects, in alphabetical order
	$args = array(
		'orderby' => 'name',
		'order'   => 'ASC'
	);
	$categories = get_categories( $args );

	// Loop through each category
	foreach ( $categories as $category ) {
		// Find posts in just this category OR find posts in this category or
		// child categories - comment out as appropriate
		$posts_query_args = array(
//			'category__in'   => $category->cat_ID, // JUST this category
			'cat'            => $category->cat_ID, // This category or a child category
			'posts_per_page' => -1, // Make all posts in a category show
		);
		$posts_query = new WP_Query( $posts_query_args );

		if ( $posts_query->have_posts() ) {
			// We have at least one post, so let's echo out the category name
			printf(
				'<h2><a href="%s" title="%s">%s</a></h2>',
				esc_url( get_category_link( $category->term_id ) ),
				esc_attr( sprintf( __( "View all posts in %s" ), $category->name ) ),
				$category->name
			);

			echo '<ul>';

			// Loop through the posts
			while ( $posts_query->have_posts() ) {
				$posts_query->the_post();
				echo '<li><a href="' . esc_url( get_permalink() ) . '">' . get_the_title() . '</a></li>';
			}

			echo '</ul>';
		}

	}

	// Now we've finished looping, reset the global $post to the data of the
	// page that our template is being used on.
	wp_reset_postdata();
}