Let me go ahead and answer the question that a LOT of people are going to be asking over the next several weeks as the release of WordPress 2.7 draws closer. Yes. Your theme, more than likely, will be compatible with WordPress 2.7.
That is to say, it won’t break under 2.7. However, this latest version of WordPress does add a good many NEW features to themes that, unless properly accounted for, your theme probably won’t be taking advantage of immediately.
For instance, threaded comments. Even if you turn on threaded comments in your new 2.7 Admin settings, unless your theme is using the new wp_list_comments() function in its comments.php file, you won’t see any threaded comments. A minor detail, but it is worth noting.
So, while 2.7 will most likely NOT break your theme, you should also keep in mind that it probably won’t be take advantage of the newest 2.7 theme features — at least not at first. Check back with your theme’s developer to see if they have plan for adding the new features into your theme in the future.