Making your theme’s comments compatible with WordPress 2.7 and earlier versions.: If you are a theme designer or author, you need to know that your legacy theme’s comments page will not be compatible with WordPress 2.7 Justin Tadlock has written up a simple (and clean) procedure to make the comments page compatible with WordPress 2.7 and still be backwards compatible with slightly older version of WordPress.
While we are on the subject of comments and WordPress 2.7, Otto has also written a detailed post on the new comment enhancements in WordPress 2.7. Judging from the comments and reading through the article, it is a very well written peice and should answer most questions on the new features.
[EDIT] As astutely pointed out in the comments, all themes should continue to work fine with original functionality. However to use the newly introduced commenting features of WordPress 2.7, older themes will have to be modified.