Straight from the horse's mouth, Microsoft's Channel9 blog has a video piece and blog post describing how IE8 is now passing the ACID2 test.
This is great news, since the included video states that they not only had to fix several bugs to do this, but there's also many CSS features they had to implement to do it.
The video is in WMV format, but it's worth a watch, since it sounds like the IE team is more standards-aware than their current browser releases would indicate :)
UPDATE: My childhood friend Jonathan Hoersch works at Microsoft, and we had a chat today about this excellent news, and he says: some of the things i worked on for acid2 include the nose (including the blue hover), borders, images (the eyes), and the hyperlink scrolling.
Here's one more excerpt from our IM conversation:
Jonathan: so you know the beginning of the video where we show the acid2 evolution?
Jonathan: its from an internal video where we had the 2001 theme in the background :-) unfortunately we couldn't include the sound
It looks like the IE8 team is working towards interoperability in this effort. Now if we can get IE6 out of the ecosystem, I think that we'll have a far happier ecosystem of web designers and developers.