Video will take the same route as audio did. Soon, it will be possible to take your entire video collection with you on your ipod, and then play it back on the ipod screen, or via the mini HMDI (please god come up with a better connection format) on the ipod. Until then, you will continue to see physical media for video.
With this you have the portable take anywhere play it anywhere format of physical media, the capacity to take your entire collection, the flexibility to play it on the ipod, or connect it to your HT, and the ability to own the content.
On Demand services, while nice, are not portable, are not widespread, and will not be viable as mainstream solutions until we get everything wireless with enough bandwidth to support it. They will remain niche rental type solutions that will gain in popularity, and may replace rentals, but will not take over physical media. The harddrive and efficient portable HD players will be the future of owning your video.
We already have devices capable of this, and once the movie studios separate their asses from their heads and make video available in a smart way, we will start to see kids hooking their ipods to their TVs and playing their TV and movies. The initial quality will be DVD or even slightly worse, but they will advance quickly. We may even see portable projectors within 5 years that can replace the TV completely and we will be faced with explaining to everyone why their wall makes a terrible projection screen, and the general public will go on about how they don't care about quality and the rest of us will sit back and bemoan the damn kids for ruining video and forcing all of us to watch crappy DVD quality compressed to 4 bit color movies on a textured cream colored wall while we cling to our precious HD/BD movies like they were made of vinyl.
Damn, that took a long time to spell out on my Ouija board.