I found the following solution in another thread (http://themeshaper.com/forums/topic/change-previous_post_link-appearance-by-filter-not-working).
To customize the content of the links try tweaking some of the values in the below filter:
function childtheme_override_previous_post_link() {
$args = array ('format' => '%link',
'link' => '<span class="meta-nav">MMM</span> %title',
'in_same_cat' => FALSE,
'excluded_categories' => '');
$args = apply_filters('childtheme_prev_post_link_args', $args );
previous_post_link($args['format'], $args['link'], $args['in_same_cat'], $args['excluded_categories']);
} // end thematic_previous_post_link
add_filter('thematic_previous_post_link','childtheme_override_previous_post_link');
You can further customize the links by removing the navigation:
function remove_post_navigation() {
remove_action('thematic_navigation_above', 'thematic_nav_above', 2);
}
add_action('init', 'remove_post_navigation');
...and adding whatever you require:
function add_custom_navigation_above() {
?>
<div id="nav-above" class="navigation">
<div class="nav-previous"><?php thematic_previous_post_link() ?></div>
<div class="nav-next"><?php thematic_next_post_link() ?></div>
</div>
<?php
}
add_action('thematic_navigation_above','add_custom_navigation_above');
This will do the trick ;-)