Konami has just confirmed that there will be a demo for the new Pro Evolution Soccer 2010, through their European PES Team Leader Jon Murphy. There's still no exact date set yet, but it's safe to bet it'll be in the next three weeks. PES 2010 comes out on October 22.
Here's the announcement from Konami-Europe's blog:
Not long to go now until we release a demo for everyone to test. You'll get a chance to see the new PES for yourselves and make up your minds about how we've reflected what the what fans want.