As you know Internet Explorer9 Platform Preview for developers was announced and released during the MIX10. Also you might be knowing that its built from the ground up and it will not support Windows XP and will run on OS as Windows Vista onwards.
Many are asking why no support for Windows XP. But people forget that ever popular XP has been there since 2001. So its not as modern and modern browsers requires modern OS.
As the IE9 FAQ says :
Q. Does Platform Preview run on Windows XP?
A. No. Internet Explorer 9’s GPU-powered graphics take advantage of new technologies available in Windows 7 and back-ported only to Windows Vista. These technologies depend on advancements in the display driver model introduced first in Windows Vista.
Now hear it from the horse’s mouth- Dean Hachamovitch, General Manager of the Internet Explorer team answers all IE9 related queries at “Channel 9 Live at MIX10: Dean Hachamovitch on IE9”
The below video excerpt just shows the IE9 and Windows XP query ,
For the full video, where Dean along with Giorgio Sardo, Technical Evangelist for Internet Explorer (the guy who built the cool IE9 platform demo app showcasing the power of HTML5 running on a GPU) answers IE9 queries, please check - Channel 9 Live at MIX10: Dean Hachamovitch on IE9