The newest platform preview is out. The summary of what's new is on this page.
I especially find this remark interesting:
They're recommending using feature detection, and not treating IE9 like IE, but treating it like webkit or gecko. If I'm not mistaken, Ext uses quite a lot of browser detection instead of feature detection. Are the Ext devs already looking into IE9 support? Maybe this is a good time to move entirely to a feature detection model in Ext?IE9’s standards support makes it much closer, for developers, to Gecko, Presto, and the different versions of Webkit than it is to IE7. We want to make the same markup work across browsers, and want feedback from developers about the issues they find offering the same markup to IE9.