No it's not.
Take a look on these very forums. I've seen many people in the threads about Android phones talk about how they got it because they wanted something newer but weren't willing to wait for the next iPhone.
When I go to buy something and am told "you can't right now" that does NOT guarantee that I'll wait for it. It changes my 2 original perceived choices (iPhone 5 or Android) into 3 choices (iPhone 4 now, wait for iPhone 5, or Android.)
Anytime you disrupt people's choice-making process you give them a chance to change their mind. And yes, you can say that that's gonna happen anyway because the iPhone 5 isn't out yet, but when you announce stuff early you force those thoughts to the front of people's minds and make them think about it. You don't want to force them into thinkig about it until you're nearly ready to sell your own product.
Apple wants them to "think about it" for 2 weeks. Not for 2 months. That makes "wait for the iPhone 5" a less desirable choice in many people's minds.