phiv

I think I found out how skip the link to the attachment page and instead, link directly from the gallery thumbnail to the full size image:

Find out function wp_get_attachment_link in your post-template.php file (located in wp-includes) and change it to look like this (one extra code line, one extra comment line):

// get an attachment page link using an image or icon if possible
function wp_get_attachment_link($id = 0, $size = 'thumbnail', $permalink = false, $icon = false) {
	$id = intval($id);
	$_post = & get_post( $id );

	if ( ('attachment' != $_post->post_type) || !$url = wp_get_attachment_url($_post->ID) )
		return __('Missing Attachment');

	if ( $permalink )
		$url = get_attachment_link($_post->ID);

	$post_title = attribute_escape($_post->post_title);

	$link_text = wp_get_attachment_image($id, $size, $icon);

	//modification introduced to force the gallery shortcode thumbnail hyperlink to the full size image directly, rather than the attachment page
	$url = wp_get_attachment_url($_post->ID);

	if ( !$link_text )
		$link_text = $_post->post_title;

	return "<a href='$url' title='$post_title'>$link_text</a>";
}

Let me know if this works for you!

I think I found out how to do it:
Find out function wp_get_attachment_link in your post-template.php file (located in wp-includes) and change it to look like this (one extra code line, one extra comment line):

// get an attachment page link using an image or icon if possible
function wp_get_attachment_link($id = 0, $size = 'thumbnail', $permalink = false, $icon = false) {
	$id = intval($id);
	$_post = & get_post( $id );

	if ( ('attachment' != $_post->post_type) || !$url = wp_get_attachment_url($_post->ID) )
		return __('Missing Attachment');

	if ( $permalink )
		$url = get_attachment_link($_post->ID);

	$post_title = attribute_escape($_post->post_title);

	$link_text = wp_get_attachment_image($id, $size, $icon);

	//modification introduced to force the gallery shortcode thumbnail hyperlink to the full size image directly, rather than the attachment page
	$url = wp_get_attachment_url($_post->ID);

	if ( !$link_text )
		$link_text = $_post->post_title;

	return "<a href='$url' title='$post_title'>$link_text</a>";
}