The PES 2012 demo is available now on the Xbox Live Marketplace.
To access the demo, head to Game Marketplace > Games & Demos > Demos > Newest > PES 2012
Alternatively, here is a direct download link on the official Xbox website. The demo is a whopping 1.88 GB in size.
Please share your impressions with us, by posting in our 2nd demo thread on our forums.