In reply to Bradley Allen.

Perhaps referring to the development log (subversion repository and browse in trac)? The links are all under the "Browse the Code" section in the left sidebar for each theme's page.

It would be good for the theme review team to develop a changelog standard. Many themes in the directory have one. You have to look for it. It can be a separate changelog text file, in a readme file, at the themes website or many other "creative" locations. We've also seen it as comments in style.css or functions.php.