Text only email template missing

You can completely customize the theme to your liking. Just copy the theme functions from forward.theme and put them in your template.php file (change the function names to phptemplate_forward_email and phptemplate_forward_postcard) and then Drupal will use your custom theme instead of my default. You can even have different email templates for each theme on your site if you so desire.

At some point I may create a set of text only themes and add a configuration option to allow you to easily switch between them, but that will be in a later release (too busy right now).