Index: wp-includes/classes.php
===================================================================
--- wp-includes/classes.php	(revision 3378)
+++ wp-includes/classes.php	(working copy)
@@ -225,6 +225,11 @@
 		if ('' != $qv['comments_popup']) {
 			$this->is_comments_popup = true;
 		}
+		
+		//if we're previewing inside the write screen
+		if ('' != $qv['preview']) {
+			$this->is_preview = true;
+		}
 
 		if (strstr($_SERVER['PHP_SELF'], 'wp-admin/')) {
 			$this->is_admin = true;
@@ -1447,7 +1452,7 @@
 }
 
 class WP {
-	var $public_query_vars = array('m', 'p', 'posts', 'w', 'cat', 'withcomments', 's', 'search', 'exact', 'sentence', 'debug', 'calendar', 'page', 'paged', 'more', 'tb', 'pb', 'author', 'order', 'orderby', 'year', 'monthnum', 'day', 'hour', 'minute', 'second', 'name', 'category_name', 'feed', 'author_name', 'static', 'pagename', 'page_id', 'error', 'comments_popup', 'attachment', 'attachment_id', 'subpost', 'subpost_id');
+	var $public_query_vars = array('m', 'p', 'posts', 'w', 'cat', 'withcomments', 's', 'search', 'exact', 'sentence', 'debug', 'calendar', 'page', 'paged', 'more', 'tb', 'pb', 'author', 'order', 'orderby', 'year', 'monthnum', 'day', 'hour', 'minute', 'second', 'name', 'category_name', 'feed', 'author_name', 'static', 'pagename', 'page_id', 'error', 'comments_popup', 'attachment', 'attachment_id', 'subpost', 'subpost_id', 'preview');
 
 	var $private_query_vars = array('posts_per_page', 'posts_per_archive_page', 'what_to_show', 'showposts', 'nopaging', 'show_post_type');
 
Index: wp-admin/post.php
===================================================================
--- wp-admin/post.php	(revision 3378)
+++ wp-admin/post.php	(working copy)
@@ -82,7 +82,7 @@
 	?>
 	<div id='preview' class='wrap'>
 	<h2 id="preview-post"><?php _e('Post Preview (updated when post is saved)'); ?> <small class="quickjump"><a href="#write-post"><?php _e('edit &uarr;'); ?></a></small></h2>
-		<iframe src="<?php the_permalink(); ?>" width="100%" height="600" ></iframe>
+		<iframe src="<?php the_permalink(); ?>?preview=true" width="100%" height="600" ></iframe>
 	</div>
 	<?php
 	break;