ben_b

It’s an incredibly ugly hack, but it gets the job done. Add the following to /wp-admin/export.php after the </tr> for the select author dropdown to let wordpress know what category you want:

<tr>
<th><label for="author"><?php _e('Restrict Category'); ?></label></th>
<td>
<select name="category" id="category">
<option value="all" selected="selected"><?php _e('All Categories'); ?></option>
<?php
$categories= get_categories();
foreach ($categories as $cat) {
	$option = '<option value="'.$cat->term_id.'">';
	$option .= $cat->cat_name;
	$option .= '</option>'; echo $option; }
?>
</select>
</td>
</tr>

and then replace the appropriate lines (from about 17 up to $categories = (array)…) in the /wp-admin/includes/export.php file to adjust the export filter accordingly.

$where = '';
if ( $author and $author != 'all' ) {
	$author_id = (int) $author;
	$where = $wpdb->prepare(" WHERE post_author = %d ", $author_id);

	// grab a snapshot of post IDs, just in case it changes during the export
	$post_ids = $wpdb->get_col("SELECT ID FROM $wpdb->posts $where ORDER BY post_date_gmt ASC");
} else if ($category and $category != 'all') {
	$posts = get_posts('category=' . $category . '&numberposts=-1');
	foreach ($posts as $post) {
		$post_ids[] = $post->ID;
	}
}

$categories = (array) get_categories('get=all');
$tags = (array) get_tags('get=all');