Not working in 2.5.1: posts_nav_link, next_posts_link, or next_post

This weekend I migrated from 2.0.11 to 2.5.1. I was surprised to find that my next_posts_link and previous_posts_link navigation system had stopped working. I use this to put “older” and “newer” links at the top of each single post, like:

<?php previous_post('%','&laquo; Older', 'no'); ?> |
<?php next_post('%','Newer &raquo;', 'no'); ?>

which formerly lived on comments.php. You can see this working on my archive blog, e.g. here.

After moving to 2.5.1 those links stopped displaying. I also tried posts_nav_link and next_posts_link, to no avail, e.g. editing my main index template to add the following inside The Loop:

<?php if (is_single()) { ?>
<?php echo "hello"; ?>
     <?php posts_nav_link(' &bull; ', __('&laquo; Newer Entry'), __('Older Entry &raquo;')); ?>
<?php } ?>

When I do this, my single posts simply say “hello.” There is no output from the posts_nav_link tag.

Out of curiosity, I loaded up the Kubrick theme. I added “there should be some nav here” such that the Main Index Template had:

<div class="navigation">
			<div class="alignleft"><?php next_posts_link('&laquo; Older Entries') ?></div>
there should be some nav here
			<div class="alignright"><?php previous_posts_link('Newer Entries &raquo;') ?></div>
		</div>

When I applied the Kubrick theme, that section of the page said “there should be some nav here.” In other words, the next_posts_link and previous_posts_link didn’t work. So I assume it’s not me doing something wrong, since Kubrick came with my 2.5.1 install and I assume it’s 2.5.1-compliant.

Is there a magic new tag I should be using? (If so, the WordPress docs should be updated.)