In previous versions, the comments_popup_link produced code that looked something like this:
<a href="pageurl/#comments" class="commentslink">Comments</a>
However, in 2.0, the same page template will produce this instead.
<a href="pageurl/#respond" class="commentslink" title="Comment on Post title">Comments</a>
Note the addition of the title attribute.
All works fine most of the time. However, when the post title contains an ampersand character, then the source shows up as something like
title="Comment on Post title with & in it"> rather than escaping the ampersand as
Where this occurs, the page will no longer validate correctly as XHTML.