WotC lists products literally months in advance. Usually at least 6. (Can't check their site at work, but amazon has Feywild for november and Vile Darkness for december... bit different than 'Ogres for september, necrons probably october or november and... uh.. probably something for that other month', isn't it?

)
Details come over time, but they usually run detailed previews with actual excerpts from a book for about 2 months before the release of the book.
They aren't going to just spring Players Handbook 4 (or whatever their current plans are) on people and put it up unannounced for sale next week.
In fact, given that GenCon is this weekend, I wouldn't be at all surprised if they announce most of next year's releases this weekend. I will in fact be very surprised if I don't know what next year's campaign setting is by Sunday (assuming I care enough to check) or indeed, if the PH 4 is coming out in 2012 or not.
But then, WotC is actually willing to spend money on this strange concept known as marketing.
Or take another example: the star wars old republic MMO. The marketing for that is going crazy, preorders are up, and are breaking sales records even though their isn't an official release date or monthly fee announced. But the game itself was announced around 3 years ago. The company actually sharing whats going on is certainly not hurting sales... in fact at the rates the preorders are going, they may break even on the development costs before the game even comes out. (Still need more to make the kind of profit they want, of course, but its getting to the point that it cannot be an abject failure in terms of money spent, minor failure financially is still possible, a failure as a game is, but really bioware, EA and lucas can go have 'we've got their money' party already)