If you read the fine print of those lowball predictions, you'll see that they actually said there would only be about 300,000 produced in March. The # was not for how many would be available. They've been in production for a while now. The only thing that qualified the one shown at the announcement as a prototype was the software. The hardware was already set in stone and well on its way. I'm guessing there will be between 800,000 and 1 million available.
If there are any shortfalls, you can rest assured that partner retailers such as Best Buy will be the ones to not get them. Apple will make sure, based on pre-orders and expected demand, that they have plenty for their retail stores.