Index: wp-cat2tag.php
===================================================================
--- wp-cat2tag.php	(revision 9771)
+++ wp-cat2tag.php	(working copy)
@@ -107,11 +107,12 @@
 <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
 
@@ -222,6 +223,19 @@
 			return false;
 		}
 	}
+	
+	function set_default_category() {
+		global $wpdb;
+		$default = get_option('default_category');
+		$allposts = $wpdb->get_results("SELECT ID FROM $wpdb->posts WHERE post_type = 'post'");
+		foreach ($allposts as $thispost) {
+			$terms = array();
+			$terms = wp_get_post_categories($thispost->ID);
+			if (empty($terms)) 
+				wp_set_post_categories($thispost->ID, $default);
+		}
+		echo __('Set default category on orphaned posts.')
+	}
 
 	function convert_categories() {
 		global $wpdb;
@@ -320,6 +334,8 @@
 			}
 		}
 		echo '</ul>';
+		
+		$this->set_default_category();
 
 		if ( ! empty($clean_term_cache) ) {
 			$clean_term_cache = array_unique(array_values($clean_term_cache));