Index: p2-new-post-categories.php
===================================================================
--- p2-new-post-categories.php	(revision 109)
+++ p2-new-post-categories.php	(working copy)
@@ -111,6 +111,7 @@
 			'orderby'    => 'name',
 			'id'         => 'p2-new-post-category',
 			'hide_empty' => false,
+			'selected'   => get_option( 'default_category' ),
 		) );
 
 		wp_dropdown_categories( $params );