This weekend, for the first time, IE 8 usage surpassed IE 6 usage in the Net Applications usage share daily trend reports.

Now, IE 6 always drops off dramatically during the weekends while most other browsers are fairly stable from weekdays to weekends, so this isn't the "real" milestone but we're probably only about a week away from the average weekly share of IE 8 beating the average weekly share of IE 6.
IE 6's share has been falling at a pretty consistent rate for more than a year now and while I don't think IE 8 has had any appreciable impact on that rate of falling, it's still nice to know that within a very short time, it will be more critical to general Web development to test in IE 8 than in IE 6 and that's good news.

This data is already a couple of weeks old, but you can definitely see the historical trends. IE 6's slide continues. IE 8 is taking share from IE 7 pretty much the same way IE 7 cannibalized IE 6 two and a half years ago.
Good times!