I have WordPress installed on my site in a WordPress subdirectory. I decided to upgrade it to WordPress 2.0 RC3 yesterday and put a post up about the upgrade.
The post-slug wound up being "wordpress-20-RC3-upgrade" and I noticed that the comments-template would not display. After checking that comments were turned on I putzed around a bit and did some additional posts. The comment template showed up fine. I then did another post called 'Wordpress Update' and found that the comments template once again would not display for this post.
I changed the word 'Wordpress' in the slugs to 'WP' and everything started working normally.
Just to give you some additional background, I have verbose rewrites on because I was initially having trouble with the redirects that I had installed in my .htaccess file. The only way I could get them to work was to go back to the old rewrite mechanism.
My site is http://www.bieberlabs.com. I have two posts on WordPress where the slug has been changed to 'wp' at the top of the blog. An older post with WordPress IN the slug can be found here: http://www.bieberlabs.com/wordpress/archives/2005/10/04/wordpress-plugin-ultimatetagwarrior/