Ok, I think I know what has happened with the upgrade to 2.3. Apparently, my wp_categories table has been renamed to wp_terms and nobody bothered to update the WP knowledge base about this little fact (at least not yesterday that I was looking for it). Good job!
I guess something like that must have happened since I was looking at my MySQL tables and I got a bit panicked when I noticed my wp_category and wp_post2category tables missing. I upgraded another WP installation I have for testing and noticed the change.
Now, for my problem with the “Zoulapia” error (which is part of my modified theme), I’ve managed to find the solution myself. I just needed to change the following in the function call.
("SELECT cat_ID FROM " . $wpdb->categories . " WHERE cat_name='" . $hemingwayEx_options['asides_category'] . "'")
("SELECT term_ID FROM " . $wpdb->terms . " WHERE name='" . $hemingwayEx_options['asides_category'] . "'")
And you can see it worked 🙂
Now I need to find out how to modify the popularity contest plugin so that it can search in the correct table when taking the popular category :-/
Oh, also the 2.3 feature that supposedly would use the previous tags you had, did not work. UTW has a (apparently) scary function to import them to [tag]Wordpress[/tag] db, but I’d rather wait a bit for that.
 UPADE: It seems they did mention it here