The 4e run is larger than the 3.5e run, according to Mike Mearls.
On the other hand, Hasbro makes a considerable amount of its money through the book trade. The book trade uses returns. The *retailers* are buying books to sell, so the question is how much of this stock is going to take the form of returns further down the line.
Now it's likely that much of the demand is driven by discount retailers online. This is good because returns are less likely. It is not so good because it generates downward pressure on pricing. You may like that, but it drives down the margins and thus, the game's profitability.
The final answer? There isn't one. It could be great! It could be not so great. Time will tell. I think the PDF will bite into the set of people who buy D&D because they used to play it and are curious, but probably won't be starting a group. On the other hand, I have a feeling that Hasbro would like to show this older, non-participant demographic the door, so to speak. So I don't think it will really affect long-term sales.
One way to boost the game is to buy it from a retailer that doesn't do returns or impose discounts. Buying from your LGS makes a more definitive economic statement, if that kind of thing matters to you.