No. Apple does not distribute it's Operating systems commercially via download. For the vast majority of consumers, downloading a 4 gig file is not as feasible as having it delivered or purchased at retail. Another thing, the download would have to be written to a CD anyway due to how the OS installers work - you can't install them if you boot off a hard drive.
Shipping a physic DVD is the most painless way to do it. Apple only offers downloads to developers who are part of the premium or select program or (I believe) if you are purchasing a volume license.
Really, downloading a 6+ gig download is very daunting for home users.