But you have to ask yourself: why so slow?
They have, by far, the greatest amount of resources out there to develop the browser, and yet the latest version of IE can only pass two tests that other browsers have all been able to pass for the past couple of years now?
Are we going to have to wait until IE10 before it passes A3? And by then Opera, Chrome, Firefox, etc, will all be working towards/already passing the Acid4 test (if there is one by then, that is).
What's more, Microsoft sites themselves work particularly bad on other browsers when compared to IE (I rarely get passed the loading-circle-thing in the center of the screen on some browsers before giving up and it instantly loading on IE). Windows Live Messenger, up until recently, forced you into loading IE whenever you clicked on something, regardless of what your default browser is.
And, let's be frank, it's not like Microsoft haven't got anything to gain from stuffing IE down our throats has it? It can use it to push it's Live services (Live search being default, isn't it?), and I wonder just how many users have MSN as their homepage. It's also good for keeping users on Windows computers, if the only software they're used to, is only available on Windows (well, it's out on some other OSs but it's a dated version).
It's not like Microsoft hasn't got into any trouble about these kinds of practises before, especially when it comes to Internet Explorer.