Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 26888)
+++ src/wp-includes/media.php	(working copy)
@@ -735,7 +735,8 @@
 		'size'       => 'thumbnail',
 		'include'    => '',
 		'exclude'    => '',
-		'link'       => ''
+		'link'       => '',
+		'class'		 => ''
 	), $attr, 'gallery'));
 
 	$id = intval($id);
@@ -804,9 +805,26 @@
 			/* see gallery_shortcode() in wp-includes/media.php */
 		</style>";
 	$size_class = sanitize_html_class( $size );
-	$gallery_div = "<div id='$selector' class='gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}'>";
-	$output = apply_filters( 'gallery_style', $gallery_style . "\n\t\t" . $gallery_div );
 
+	//allow plugins or themes to alter the gallery class
+	$gallery_class = apply_filters( 'gallery_class', "gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class} {$class}", $selector, $attr );
+
+	//allow plugins or themes to alter the gallery container's opening markup. E.g. include extra attributes or even use a different tag
+	$gallery_start = apply_filters( 'gallery_container_start', "<div id='$selector' class='$gallery_class'>", $selector, $gallery_class, $attr );
+
+	//allow plugins or themes to alter the default gallery inline styles
+	$gallery_style = apply_filters( 'gallery_style', $gallery_style, $selector, $attr );
+
+	//allow plugins or themes to alter the gallery column separator markup
+	$gallery_separator = apply_filters( 'gallery_column_separator', '<br style="clear: both" />', $selector, $attr );
+
+	//allow plugins or themes to alter the gallery container's closing markup
+	$gallery_end = apply_filters( 'gallery_container_end', "
+			<br style='clear: both;' />
+		</div>\n", $selector, $attr );
+
+	$output =  $gallery_style . "\n\t\t" . $gallery_start;
+
 	$i = 0;
 	foreach ( $attachments as $id => $attachment ) {
 		if ( ! empty( $link ) && 'file' === $link )
@@ -816,6 +834,7 @@
 		else
 			$image_output = wp_get_attachment_link( $id, $size, true, false );
 
+		$image_output = apply_filters('gallery_image_output', $image_output, $id, $size );
 		$image_meta  = wp_get_attachment_metadata( $id );
 
 		$orientation = '';
@@ -835,12 +854,10 @@
 		}
 		$output .= "</{$itemtag}>";
 		if ( $columns > 0 && ++$i % $columns == 0 )
-			$output .= '<br style="clear: both" />';
+			$output .= $gallery_separator;
 	}
 
-	$output .= "
-			<br style='clear: both;' />
-		</div>\n";
+	$output .= $gallery_end;
 
 	return $output;
 }