<?php
$employeeQuery = new WP_Query(array('post_type' => 'professionals', 'meta_key' => 'prof_lastname', 'orderby' => 'meta_value', 'order' => 'ASC')) ?>
    <?php $prevltr = ""; ?> /* holder variable so we know what our current letter is */
<?php if(have_posts()) { while ( $employeeQuery->have_posts() ) { $employeeQuery->the_post(); ?>
    <?php $lastname = get_post_meta($post->ID, 'prof_lastname'); ?> /* get the last name post meta */
    <?php $curltr = strtoupper(mb_substr(trim($lastname,0,1,'UTF-8')))?>
    <?php if($curltr != $prevltr) : ?> /* is our current letter different? */
      <?php echo $curltr; $prevltr = $curltr; ?> /* print out the current letter and update prevltr */
    <?php endif; ?>

                  <article class="professional-entry" id="entry-<?php echo $last_char ?>">

                      <div><strong><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></strong></div>
                      <div><?php $terms = get_the_terms( $post->ID , 'titles' ); foreach( $terms as $term ) { print $term->name; unset($term); } ?></div>
                      <div><a href="mailto:<?php echo get_post_meta($post->ID, 'prof_email', true); ?>"><?php echo get_post_meta($post->ID, 'prof_email', true); ?></a></div>

                  </article>

              <?php }} // End the loop. Whew. ?>