For the first time ever, Microsoft will be offering the final retail versions of a new OS as a download. Vista will be available to be purchased, same price as in the stores, and then downloaded direct from the Windows Marketplace. All other licensing remains the same, the only difference being that you no longer get the official media with it. While less costly for Microsoft, they aren't passing the savings on to you – which make sense, because they are selling licenses, not media.
The article is speculating that it may help spur further sales of Vista, with the ease-of-acquisition coming in to play.