append-link-to-rss-content.php

<?php
// Appends a "View original at http://yourwpblog" link to end of post content
// in WordPress RSS feeds

function append_to_feed($postContent) {
        global $post;

        // append the link to the post content
        $postContent .= '<p>(<a href="'.get_permalink().'">View original at '.get_bloginfo('url').'</a>)</p>';

        echo $postContent;

}


function run_append_filter() {

    if (is_feed()) {
        // you need both because the_content_rss isn't applied
        // to rss2 & atom. Yes, this is a bug: 
        // http://trac.wordpress.org/ticket/3884
        add_filter('the_content', 'append_to_feed');
        add_filter('the_content_rss', 'append_to_feed');
    }

}

add_action('template_redirect', 'run_append_filter');