Tested on 2 installations using TwentyTen. If your site is set to use a static front page and you opt to run a search from (say) the Home page, the current_page_parent class is added to the main posts page. If you run a search from a sub-category, the posts page get the current_page_ancestor class.
I only spotted this because I was working on a theme that applied different background colors to current_page_parent and current_page_ancestor. In TwentyTen, you need to check the source markup.
This class application seems illogical to me. A search results page has no direct connection with the main posts page. This is especially true for those who are using WP as a CMS with perhaps only a single catgeory of news-type posts. if anything, these classes should be applied to the static front page.
It's as if the core devs confused is_front_page with is_home, if you know what I mean...