Displaying caption with featured image

To use this you will need to add this to in place of your themes thumbnail function:

function your_thumbnail_caption($html, $post_id, $post_thumbnail_id, $size, $attr)
{
$attachment =& get_post($post_thumbnail_id);

if ($attachment->post_excerpt || $attachment->post_content) {
$html .= '<p class="thumbcaption">';
if ($attachment->post_excerpt) {
$html .= '<span class="captitle">'.$attachment->post_excerpt.'</span> ';
}
$html .= $attachment->post_content.'</p>';
}

return $html;
}

add_action('post_thumbnail_html', 'your_thumbnail_caption', null, 5);

Your current call to the post thumbnail should work, but just in case here is the code I use in the index.php or the post.php (whichever your theme is using).

<?php the_post_thumbnail(); ?>

Now when you add a post thumbnail you can type your caption for the image. Make sure you save the image after adding the thumbnail. You can also add captions to existing thumbnails by going to the post edit screen, select the post and click the current featured image to bring up the image caption area.