I had a problem where after I upgraded to 3.0.1, people were getting a blank page when submitting a comment, and the comment wasn't added.
The comment form was on a custom page template.
As it turns out, if you use the default theme from 2.9.2 and below, you will need to add The Loop above the call to comment_id_fields(); or else the post id will be blank in the form and therefore not a valid submission.
Adding The Loop to other files from old themes has fixed other problems as well.
I hope this helps someone.