Query to retrieve post id’s under a specific category

Hi

My programming skills are a little poor and i’m not having much luck with google or the wordpress documentation on this. What i’m trying to do is inside a loop, retrieve all of the post’s ids for posts inside that category, and get information from each of those posts custom fields, then display them in a featured content box.

Here is the code I have at the moment, i dont seem to be able to parse $post->ID, the code works if I input a post ID, but I cant seem to get this code to output a list of the ID’s and then display the 3 posts

<?php
$my_query = new WP_Query('category_name=featured&showposts=3');
while ($my_query->have_posts()) : $my_query->the_post();
$do_not_duplicate = $post->ID;
?>
<li class="sliderImage">
  <img class="featimg" src="<?php echo get_post_meta($post->ID, 'thesis_image', $single = true); ?>" alt="<?php the_title(); ?>"  />
  <span><?php the_excerpt(); ?></span>
</li>
<?php endwhile; ?>