Bront
The man with the probe
He actualy said shortly after the final book, not beforeWik said:I think KO is right in that a functioning world released just before the final book is out would be a good idea. That being said, I also think it's great to discuss this subject long before we actually put pen to paper.

It's an important difference, and one I fully support.
With the PHB, you can build a very vague basis. The DMG can help you design the initial starting area (an important but medium sized trade city is a good start, much like Orussus). And once the Monster Manual is out, you can flesh out some finishing touches and start her up.
A good reason to wait that long is to allow PHB circulation. Especialy if there's a delay in getting them out (like the Mutants and Masterminds 2.0 books, which sold out so fast many weren't able to get them till 4 months later). With WotC, it's less likely to happen, but I know I might not get a book myself the first week it's out.