While I agree that Blockbuster and Netflix should be worried, I don't think Apple will be driving physical DVD rental stores out of business anytime soon. The one advantage that Blockbuster has is bandwidth. Movies, if you want them to look good on large screens, take up a lot of space. That means whether you're streaming them or downloading them you need a pretty fat pipe to have an enjoyable experiences. Compare this with the rather small files that most songs, in MP3 format, create and you can see how the music business was greatly impacted by digital distribution whether it be legally via iTunes, or when the floodgates really opened during the freewheeling days of Napster (I was an undergrad during that time, and I can tell you that I saw many a computer running Napster. I, of course, never downloaded anything because I didn't actually own a computer in college). Theoretically it could take less time to drive to a Blockbuster and rent a DVD than it would to download the movie. This will become less of a problem, and digital rentals more popular, when broadband speeds make downloading multi-gigabyte files take a matter of moments (in some areas this is already true).
Clearly digital movie distribution, both rental and for purchasing, is the future, but sadly I think this future is still a few years off from supplanting those shiny disks we all know and love.