Upgrading to WordPress 2.0 brings many advantages but the installation may create some page not found, 404, errors for you.
If you see these when you have previously established a permalink structure, and you are certain that the pages exist, then you should check your htaccess file.
By default WordPress tries to write to this file to enable the rewrites that will conform to the permalink structure you have defined.
On some hosting accounts WordPress may not have the right permissions to edit the file and the rewrite functionality will fail. The rules will relate to WordPress 1.5 whereas you will be running WordPress 2.0.
In these situations you should edit the htaccess file manually, if you are confident of doing this and not shutting down your blog – take a backup.
WordPress 1.5 seemed to produce a very large set of rewrite rules whereas WordPress 2.0 handles rewrites differently and the rules are fewer. So do not worry if the replacement code is much shorter than the original code.
Good luck and I hope that this helps.