Slashdot has written an interesting detail on the new Acid browser test. It seems like, according to the results compiled in this page, Internet Explorer 5.5 beats 6.0 and 7.0 on Acid3.
The World Wide Web Consortium (W3C) develops standards for Internet applications. For example, XHTML, HTML, CSS, etc. Acid 3 basically takes a lot of W3C"s newer standards and tests to be sure that the browser supports the features it should and that it behaves as it should when using them. Acid 3 is a suite of 100 different tests to be sure that these standards are being met.