Needless to say, this is the first time a download retailer has taken the number one spot and represents a notable market share gain for Apple even since February. It will now be interesting to see if Apple can hold on to the top spot in the long run. In any case, this is a watershed moment and brings the mainstream death of the CD that much closer into view.
Update: Apple has since confirmed its position with a press release. "The iTunes Store became the largest music retailer in the US based on the amount of music sold during January and February 2008."