I just tested Acid test 3 on my fresh installed IE9 beta with all default settings. IE9 beta scored 95 out of 100. I hope it will be able to score 100 / 100 in its final version.
What is Acid test
Acid3 is a test page from the Web Standards Project that checks how well a web browser follows certain selected elements from web standards, especially relating to the Document Object Model (DOM) and JavaScript.
Acid Test 3 url: Acid Test 3