Re: occam and Scribble's last two posts - exactly. Any new "editions", per se, will be more of "state of the rules" updates, snapshots of where the living D&D system is at the moment. This is why I see a revised PHB as a good thing and almost inevitable.
DDI allows WotC to spit out new variations of presentation, angled at different markets, rather than new editions. Essentials might give them a bump but I'm sure they're already trying to figure out what their next big marketing wave. I think the smart thing to do would be to make DDI so good that everyone wants it. Flesh out the Adventure Tools with a variety of Builders - NPCs, Dungeons, Encounters, even Classes, Feats, etc. Create a tiered subscription (even with Heroic, Paragon, and Epic as names) so that it is very cheap to have basic access, but more expensive the more tools you want, with exclusive content for the "Epic" subscribers.
DDI can keep on evolving, keep on filling out and tweaking the rules through micro-changes. There wouldn't need to be a true "5E" unless D&D transformed completely. But there could be new physical presentations of the core rules every few years as they try to meet different markets. First we had the 4E core rulebooks, now we have the Essentials product line; in a few years it could be something completely different, or it could be a revision of the original core. It might be that the basic framework of the Essentials box sets is a glimpse into the future in that they offer something you can't get through DDI: bits and pieces, physical toys that we game-geeks love. As much as I like hardcovers they simply don't offer that.
But no matter what happens in the future, I think there will always be at least hardcover versions of the Big Three. Talk about iconic and a sacred cow. One would hope!