Index: wp-admin/includes/class-wp-upgrader.php
===================================================================
--- wp-admin/includes/class-wp-upgrader.php	(revision 18462)
+++ wp-admin/includes/class-wp-upgrader.php	(working copy)
@@ -1454,4 +1454,10 @@
 			$this->package = $uploads['basedir'] . '/' . $this->filename;
 		}
 	}
+
+	function cleanup() {
+		if ( file_exists($this->package) )
+			return @unlink($this->package);
+		return true;
+	}
 }
\ No newline at end of file
Index: wp-admin/update.php
===================================================================
--- wp-admin/update.php	(revision 18462)
+++ wp-admin/update.php	(working copy)
@@ -140,6 +140,8 @@
 		$upgrader = new Plugin_Upgrader( new Plugin_Installer_Skin( compact('type', 'title', 'nonce', 'url') ) );
 		$upgrader->install( $file_upload->package );
 
+		$file_upload->cleanup();
+
 		include(ABSPATH . 'wp-admin/admin-footer.php');
 
 	} elseif ( 'upgrade-theme' == $action ) {
@@ -242,6 +244,8 @@
 		$upgrader = new Theme_Upgrader( new Theme_Installer_Skin( compact('type', 'title', 'nonce', 'url') ) );
 		$upgrader->install( $file_upload->package );
 
+		$file_upload->cleanup();
+
 		include(ABSPATH . 'wp-admin/admin-footer.php');
 
 	} else {