if you are using a program like notepad++ (which you should, i recommend it as a kick ass text editor) then you can just search content-extensions.php for 'thematic_post'. you'll find the filter is in the thematic_content() function. i originally thought that the filter name was the same as the function, but this isn't the case. the filter name is what is seen in the apply_filters(). here's the source:
function thematic_content() {
global $thematic_content_length;
if ( strtolower($thematic_content_length) == 'full' ) {
$post = get_the_content(more_text());
$post = apply_filters('the_content', $post);
$post = str_replace(']]>', ']]>', $post);
} elseif ( strtolower($thematic_content_length) == 'excerpt') {
$post = '';
$post .= get_the_excerpt();
$post = apply_filters('the_excerpt',$post);
if ( apply_filters( 'thematic_post_thumbs', TRUE) ) {
$post_title = get_the_title();
$size = apply_filters( 'thematic_post_thumb_size' , array(100,100) );
$attr = apply_filters( 'thematic_post_thumb_attr', array('title' => 'Permalink to ' . $post_title) );
if ( has_post_thumbnail() ) {
$post = '<a class="entry-thumb" href="' . get_permalink() . '" title="Permalink to ' . get_the_title() . '" >' . get_the_post_thumbnail(get_the_ID(), $size, $attr) . '</a>' . $post;
}
}
} elseif ( strtolower($thematic_content_length) == 'none') {
} else {
$post = get_the_content(more_text());
$post = apply_filters('the_content', $post);
$post = str_replace(']]>', ']]>', $post);
}
echo apply_filters('thematic_post', $post); //here's your filter right here
}