Some of the other posters have hit it right on the head... I think a new edition will be announced as soon as the company decides that it's a good idea to do so. Their criteria won't necessarily match those of the entire fan base, however, and even if they did, people won't agree as to whether those criteria have been reached or if they are even valid.
I get the feeling that D&D in its current incarnation will have a lifespan of approximately five years. I don't have any reason for this, it's just a gut feeling.
For my own part, I hope that D&D never goes to an entirely digital method of distribution. I'm done with hauling a laptop back and forth between game sessions, and I don't want technology to "improve" my game any further. I'm sure that this isn't everyone's experience... But 4E players that I've played with know less about the game, less about the rules, and less about what their characters can do than in any version that I've ever seen. At first, I thought it was just my group. But two years in, playing on and off with eleven separate groups, many players still think that "I don't have to know that, the character generator does that for me."
I play 4E. But I do think technology enables some people to become lazy. Does anyone remember when cellular telephones weren't common? People used to actually memorize phone numbers, not because they tried, but because they dialed them so much. Remember when GPS technology wasn't available? People used to be able to give you directions to somewhere you haven't been before. Nowadays, people don't commonly know any phone numbers (except, perhaps, their own), and asking someone for directions usually gets you told to go to Google. That same phenomenon has been creeping into the 4E games I've played within the last two years. People don't know their material and don't want to learn it because they expect technology to do it for them.
Despite the rules being simpler and the character options far more limited, players know the rules and how their characters work to a lesser degree than in previous editions.