Thought this http://arstechnica.com/microsoft/news/2010/03/ie9-standards-... was interesting. It paints Miscrosoft as a company that aims to thoroughly support standards, although what they may be doing is looking for under-specified parts of the specification and defining the standard in those areas.
It's one of many tests. While they aren't at 100/100 yet and rendering the page correctly, they have made solid strides since their last release of IE9. They are progressing.
But yeah, ACID3 isn't the only test out there browser makers appear to use.
