If you have invested some time in creating an index.php file and you don't want to use themes, here's an easy way to make 1.5 work much like 1.2 did in editing templates.
The wp-admin/templates.php file is still in 1.5 so even if you've done a fresh install, all you have to do to get to a new version of the Templates page from 1.2 is point your browser at http://yoururl/wp-admin/templates.php
Now, manually typing the url gets old, so what you need to do is edit the wp-admin/menu.php file to add a listing for templates.php.
$menu = array(__('Dashboard'), 0, 'index.php');
$menu = array(__('Write'), 1, 'post.php');
$menu = array(__('Manage'), 1, 'edit.php');
$menu = array(__('Links'), 5, 'link-manager.php');
$menu = array(__('Presentation'), 8, 'themes.php');
$menu = array(__('Templates'), 8, 'templates.php');
$menu = array(__('Plugins'), 8, 'plugins.php');
$menu = array(__('Users'), 0, 'profile.php');
$menu = array(__('Options'), 6, 'options-general.php');
I have added the line with array item #27 to show the templates.php file in the menu header of the wp-admin/index.php file.
I thought I'd broken something at first 'cause when I hit 'update file' I got an error about headers already being sent, but after a reload or two it straigtened out and works a treat.