Question: I want a writing panel for a custom field called 'Sub-title' and would like it to appear in its logical place in the Page editor (that is, between the title and the Page content). Is that possible? 

I would like to avoid having to create a custom post type, since a Page + custom fields is all I really need.

Answer: 
I am wondering why my first answer was down-voted. :)

I think the only way to do is to add a meta box below the post content field and use javascript to move it above.

This is the way it is done with the slug field in the WordPress core (the slug field is were you edit the page or post url). Disable javascript in your browser and you will see the slug field at the bottom of the page.
If WordPress does it with javascript, I think the best way to do it is javascript too. As, I can't see any hook between the title and content fields which will allow it to be done without using javascript.
I'd like to also suggest the use of [WPAlchemy Meta Box](http://farinspace.com/wpalchemy-metabox/) for creating your meta boxes ...

Then do something like:

```
$custom_metabox = new WPAlchemy_MetaBox(array
(
    'id' => '_custom_meta',
    'title' => 'My Custom Meta',
    'template' => TEMPLATEPATH . '/custom/meta.php',
    'lock' => 'after_post_title', 
));

```