This is great news from MS. In the past, their attitude has seemed to be "Standards? We have 99%+ market share. We ARE the standard."
With IE9 that seems to be changing. They have posted the Acid3 test results on the preview page. They list things (DOM events, CSS3 selectors, HTML5 compatibility) that they will be supporting with IE9.
Web developers have to be loving this. It will make their lives so much easier... especially considering that IE6 was "buried" this year, lol.