Index: wp-admin/import/wp-cat2tag.php
===================================================================
--- wp-admin/import/wp-cat2tag.php	(revision 11001)
+++ wp-admin/import/wp-cat2tag.php	(working copy)
@@ -108,11 +108,12 @@
 <ul style="list-style:none">
 
 <?php	$hier = _get_term_hierarchy('category');
+		 $default = get_option('default_category');
 
 		foreach ($this->all_categories as $category) {
 			$category = sanitize_term( $category, 'category', 'display' );
 
-			if ( (int) $category->parent == 0 ) { ?>
+			if ( ((int) $category->parent == 0 ) && ( $category->term_id != $default )) { ?>
 
 	<li><label><input type="checkbox" name="cats_to_convert[]" value="<?php echo intval($category->term_id); ?>" /> <?php echo $category->name . ' (' . $category->count . ')'; ?></label><?php
 
@@ -224,6 +225,52 @@
 			return false;
 		}
 	}
+	
+	function set_default_category() {
+		global $wpdb;
+		
+		$default_cat_id = (int) get_option('default_category');
+		
+		if ( !$default_cat_id )
+			return;
+		
+		$default_cat = get_term_by('term_id', $default_cat_id, 'category');
+		$tt_ids = array($default_cat->term_taxonomy_id);
+		$terms = array($default_cat->term_id);
+		
+		# mass fetch ids
+		$post_ids = $wpdb->get_results("
+			SELECT	posts.ID
+			FROM	$wpdb->posts as posts
+			LEFT JOIN $wpdb->term_relationships as term_relationships
+			ON		term_relationships.object_id = posts.ID
+			AND		term_relationships.term_taxonomy_id = $default_cat->term_taxonomy_id
+			WHERE	post_type = 'post'
+			AND		term_relationships.object_id IS NULL
+			");
+		
+		# mass insert relationships
+		$wpdb->query("
+			INSERT INTO $wpdb->term_relationships ( object_id, term_taxonomy_id )
+			SELECT	posts.ID, $default_cat->term_taxonomy_id
+			FROM	$wpdb->posts as posts
+			LEFT JOIN $wpdb->term_relationships as term_relationships
+			ON		term_relationships.object_id = posts.ID
+			AND		term_relationships.term_taxonomy_id = $default_cat->term_taxonomy_id
+			WHERE	posts.post_type = 'post'
+			AND		term_relationships.object_id IS NULL
+			");
+		
+		# update count in one go
+		wp_update_term_count_now($tt_ids, 'category');
+		
+		# mass do_action
+		foreach ( $post_ids as $object_id ) {
+			do_action('set_object_terms', $object_id, $terms, $tt_ids, 'category', false);
+		}
+		
+		echo __('Set default category on orphaned posts.')
+	}
 
 	function convert_categories() {
 		global $wpdb;
@@ -322,6 +369,8 @@
 			}
 		}
 		echo '</ul>';
+		
+		$this->set_default_category();
 
 		if ( ! empty($clean_term_cache) ) {
 			$clean_term_cache = array_unique(array_values($clean_term_cache));