According to the IE Blog, IE8 officially passes the Acid2 tests now. You can see it in action over on Channel9 and hear more about IE8 in an interview with the IE team.
From the sounds of it, you will need to opt into using the new rendering engine for backwards compatibility reasons. This will probably be done using some form of header tag in the HTML document. If it is indeed a completely new engine, that also means they essentially could design the entire version of that engine to be 100% compliance based with no backward compatibility at all. Meaning not only in terms of rendering, but also DOM/JavaScript support. Let's hope that's the case!