How to put placeholder text in the main post input area?

You can change ‘post’ to which ever post type you want to apply the default content to.

add_filter( 'default_content', 'wpse57907_default_content', 10, 2 );
function wpse57907_default_content( $content, $post ) {
    if ( 'post' == $post->post_type )
        $content="Use this area to write an introduction to the course...";

    return $content;
}

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Edit

I’m not sure you can add placeholder text to tinymce editors. The followng method will add a placeholder to the underlying textarea (you can view it on the ‘html’, soon to be ‘text’ tab):

add_filter('the_editor','wpse57907_add_placeholder');
function wpse57907_add_placeholder( $html ){
    $html = preg_replace('/<textarea/', '<textarea placeholder="my place holder text" ', $html);
    return $html;
}

But this does not appear on the visual editor (as I say, I’m not sure placeholders work with TinyMCE – if someone knows how, please comment.).