Adobe Product Manager Tom Hogarty said in a blog post that the Photoshop team would like to provide advanced notice that Photoshop CS6 (13.0) will be the last major version of Photoshop to support Windows XP. He said that modern performance-sensitive software requires modern hardware graphics interfaces that Windows XP lacks, in particular a way to tap into the power of GPUs. By only working on newer operating systems and hardware Adobe can bring in significantly better performance.
Photoshop CS6 already demonstrates that relying on a modern operating system, graphics cards/GPUs and graphics drivers can lead to substantial improvements in 3D, Blur Gallery and Lighting Effect features not available to Windows XP customers, he said.
Adobe hopes that by providing this information early it will help you understand our current decisions around operating system support and where we we're headed with future releases of Photoshop. It is hard to see how any serious user of Adobe products could be using an XP machine anyway. The move away from XP started with CS 5 which only ran on Vista.