Lorelle has a list of template and function differences between WP2.0.x and WP2.1. They are quite substantial.
Most importantly, WordPress 2.1 requires a server that is running MySQL 4.x. MySQL 3.x will not work, and will cause error messages to be displayed.
If you have any questions about upgrading, please contact us. We do all WP upgrades upon request, at no charge to customers.