I ended up having to write my own script, actually. On the main site page (index.php) I just swapped out the part of the loop referring to "the_content" with "the_excerpt":
<div class="post-content">
<?php the_content(); ?>
</div>
Becomes
<div class="post-content">
<?php the_excerpt(); ?>
</div>
Then I changed the post page (single.php) so that, for the first 30 days a post is up, everyone can see it. After 30 days, you need to be registered and logged in (authenticated) to see anything but the excerpt:
<div class="post-content">
<?php the_content(); ?>
</div>
Becomes:
<div class="post-content">
<?php global $user_ID;
get_currentuserinfo();
?>
<?php $today=date(z); ?>
<?php $postday=the_date('z', '', '', FALSE); ?>
<?php if($today - 30 > $postday && $user_ID == '') {
echo '<p style="font-size: 36px;"><center><a href="<?php bloginfo('url'); ?>/wp-login.php?action=register">Please Register!</a></center></p>';
echo '<p style="font-size: 11px; font-family: Arial, Helvetica, sans-serif;">You must be a registered user to view this post. Until you complete your registration, you will only be able to view an excerpt:</p>';
the_excerpt();
}
else {
the_content();
}
?>
<!--<?php the_content(); ?>-->
</div>
I hope that helps!