Disabling the Cookies for Comments plugin broke the comments form.
On the recommendation of Matt Cutts, I added the Cookies for Comments plugin to this blog, and also added their recommended .htaccess modifications:
RewriteCond %{HTTP_COOKIE} !^.*XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.*$
RewriteRule ^wp-comments-post.php - [F,L]
When I realized that Cookies for Comments was slowing down my blog considerably, I disabled the plugin, but did not think to remove the .htaccess rewrite rule. The remaining .htaccess rules broke the comments on this blog (for two weeks now, could it be!?).

Katz Web Services is a
Blog Comments Now Pass Juice – No More `nofollow`
January 14, 2010
in Blogging
After lots of back and forth, I have decided to make comment links dofollow (removing nofollow); meaning that links users have in their comments will be followed by search engines. This adds value to the recipient’s site.
Please note:
Enjoy, don’t abuse.
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