NOTE: This module *may* be deprecated in Drupal 7 !
Here's your reason, basically the theming changes in D7 mean the field theming gets bound a lot lot later in the process, so it's not possible to generate the output into a Textarea for you to edit - that said, it is possible that we can come at this from another angle and use tokens instead BUT it means what you see in contemplate admin page will be nothing at all similar to the output.
You can view more on taking this module forwards here http://drupal.org/node/1285558
The Content Template (a.k.a. ConTemplate) module allows modification of Drupal's teaser and body fields using administrator defined templates. These templates use PHP code and all of the node object variables are available for use in the template. An example node object is displayed and it is as simple as clicking on its properties to add them to the current template.
This module was written to solve a need with the Content Construction Kit (CCK), where it had a tendency toward outputting content in a not-very-pretty way. And as such, it dovetails nicely with CCK, adding a "template" tab to CCK content-type editing pages and pre-populating the templates with CCK's default layout. This makes it easy to rearrange fields, output different fields for teaser and body, remove the field title headers, output fields wrapped for use with tabs.module (part of JSTools), or anything you need.
But Content Template can actually be used on any node type and allows modification of the teaser and body properties before they go out in an RSS feed, search index, or are handed off to the theme. Additionally, ConTemplate can choose to add any files, images, or other media to RSS feeds as an attachment.
Templates are normally stored in the database but it is also possible to create disk-based templates which live in your sites directory and can be inherited across multisite installations in the same way as themes or modules.
After enabling the 'Content Templates' module, visit admin/content/types/templates to start editing your content templates.