save the main current post id into a variable and compare it in the loop with the current post id; example:
<?php $this_post = $post->ID; ?>
<?php $temp_query = $wp_query; ?>
<?php foreach(get_the_category() as $category) {
$cat = $category->cat_ID;
}
query_posts('orderby=date&cat=" . $cat . "&order=desc&posts_per_page=-1');
while( have_posts() ) : ?>
<span<?php if( $this_post == $post->ID ) { echo ' class="current"'; } ?>>
/*output of your posts here*/
</span>
<?php endwhile;
?>