According to this article, Internet Explorer 9 will not work on Windows XP. I'm not sure how many people using Windows XP will even care but there you have it, no IE9 soup for you.
"Building a modern browser requires a modern operating system," Internet Explorer General Manager Dean Hachamovitch said at a press conference at the Mix10 event.
http://news.cnet.com/8301-13860_3-20000561-56.html