bump... I'm seeing the same problem.
As part of a custom theme idea, I'd like to use the 'comments popup' functionality, specifically using the ?comments_popup=(ID) option, to display comments and accept new ones via an iframe.
I have an old local installation of v2.1, and it's working absolutely fine.
But I've tried several installations of 2.3.x, as well as v2.5RC1 and even wordpress.com, and none of them work.
As 5115 described, when you submit a comment, you get a blank screen back. The browser address box shows 'wp-comments-post.php'.
(The comment doesn't seem to be received properly; it certainly doesn't show on the admin dashboard.)
Does this mean 'comments_popup' is now redundant functionality?