WordPress 3.0 is going to see some major changes to design and functionality. Along with this move will be the need/desire to stop supporting some legacy features of older versions of WordPress.
This is nothing new. Just recently support for UTF (Ultimate Tag Warrior) Tag import was dropped. Most have long ago imported their UTF tags into the core of their site. Removing this support was of little consequence.
But a bigger changes in coming for sites using older themes based on the default theme.
WordPress Updates Default Theme for Twenty Ten
The new default theme for WordPress in version 3.0 is called “Twenty Ten” It is slick. I’ve played with it and I love the code and styling of the new default theme.
But the arrival of this new theme means support for some functionality based on the old default theme is going away. It means you could find your site not working like “right” after you upgrade to 3.0 if you are using one of these older coded themes.
Here is a link to a post going into detail about the changed being made and what will need to be done to make older themes based on the default theme function under the changes.