A couple of days ago, Internet Explorer developers have announced that IE 9 will now support all the CSS 3 selectors. That includes also a pseudo-class like :selection:
However, we have to wait until the final release of IE 9 will be shipped to see if their implementation is correct. In fact, claiming that IE 9 passes all the tests of css3.info doesn't necessarily mean that it's bug-free. In the meantime, stay tuned with the CSS Test Suite to get the latest test cases from Microsoft.. but wait, they actually provided no tests on CSS 3 selectors! See http://www.w3.org/Style/CSS/Test/CSS3/Selectors/current/ for more details.