The default RSS setup for WP 1.5.2 is broken. It took some digging, but I eventually found how to fix it.
1. Find footer.php (probably in wp-content/themes/default/)
2. where it says:
<a href="feed:<?php bloginfo('rss2_url'); ?>">Entries RSS</a>
replace the cryptic text inside the quotes with this:
3. For example, I replaced it with this:
4. Reload your site, and click on the Entries RSS link at the bottom of the page. If you get some ugly XML, it’s working!