It happens every time a new version of a RPG is announced - the old one becomes almost invisible on a store's shelves. The same thing happens in video games. When the next platform is announced, sales of the current platform dry up completely. The difference, in this case, is that typically the next edition isn't announced a year in advance.
Wizards continues announcing and releasing new products for 4th edition... but I can't imagine that they'll actually sell any of it. I know if I owned a comics/gaming store I sure wouldn't buy any of that dead product for my shelves.
This whole move just smacks of desperation, frankly. I think it's going to backfire on Wizards in a huge way - they just guaranteed flat sales for an entire year. I'd look for D&D to be in new hands within the next fives years. And you know what that means, don't you? A new edition!