I'm happy to say that my themes [tag]sunset[/tag] and [tag]jellyfish[/tag] are all good for the new WP2.1. I even upgraded my site without deactivating the theme. The only tweak you need is on the sidebar codes for the display of links if you have more than one categories to display. That could be tricky see this post here and this thread in the forum.
Good luck upgrading.