I looked through the WP SQL database, and I see that there is a field for user_id in in the wp_comments table. But it's always set to 0 for all comments, even my own when I write them while logged in. (recent WP 1.5 nightly)
Is there a way to store the user ID in the comment table? I'd like to use that to style comments differently without having to resort to matching the user's email.