Microsoft said at its MIX developer conference in March that Internet Explorer won't be offered for XP. The reason is that IE 9 is a "Modern browser", and getting the benefits of the hardware acceleration and other performance gains that it will offer requires a "modren OS".
Issue is the adoption of HTML 5, an upcoming repair of the Web's markup language that will add significant new capabilities, including standard ways to implement video, animation, audio and offline storage that are not included in the current HTML standard.
But If IE 9, which will be Microsoft's first browser to fully support HTML 5, isn't available for XP, then those users will be outside of that HTML 5 compatibility sphere.