At yesterday's WWDC press conference, Phil Schiller, Apple's senior vice president of worldwide marketing, claimed that the new MacBook Pro 15 with Retina display would ship immediately. It appears that that is now not the case and that could be due to a few reasons and it's not clear which reason it is.
The first reason that they could not be shipping today is some sort of miscommunication or hiccup in the shipping. Although Apple is usually good at avoiding things like this, they aren't immune singularly immune from it. Or, Apple could have sold out the first batch and are now having to make more. This is a bit more likely.
Or, since Apple is all about marketing, this could be a marketing ploy. Apple could be making it look like they sold out to help drive sales. After all, the new PC starts at a hefty $2,199 which makes it unaffordable for quite a large part of the market. But then again, Apple has never been known to make affordable products. All we know is that the website is now quoting 2-3 weeks for shipping time.