I have just upgraded from WordPress 15.1.2 to 2.0.4. Almost everything went smoothly, but I have a comment - related problem. When a user posts a comment, he/she is redirected to the "Nothing was Found" page. The comment is recorded for moderation in case of new users or posted immediately for registered users (which is ok according to my settings).
After the upgrade I also changed from "?p=123" links to title-based links. When I change back, the problem disappears, but I'd like to keep that setting without getting errors :)