To see everything that IE9.js supports visit the IE7.js Test Pages list and take your pick. The work that Dean has done is absolutely amazing and I am very happy that because of him I don't really have to worry about my websites in older versions of non-standards compliant browsers.
Even Microsoft sent flowers to the fake IE6 funeral with a note saying that we should stay tuned till this years upcoming MIX conference. I hope they say they are support HTML 5 and CSS 3 as that would be simply amazing. Maybe, just maybe they are switching from the old Trident based rendering engine to something more modern.
While I may dislike IE 6 and 7, I won't yet go so far as to put up a message just yet stating that people should get a better browser, I am removing a lot of stress and work in that I no longer have to support the old ageing browser.