With the PS2 (I can't remember if it was in Japan or America, I think the same thing happened in both Japan and America), the PS2 hype-machine was at incredible rocket-ship levels, and Sony said that they were going to explode onto the scene with a million units (in a single territory, not worldwide) on launch day.
Then like a week before the launch, Sony said "Oh wait, did we say a million? Sorry. We can't do a million. We'll have 500,000 (although there will be a million before Christmas). OMG! There's a shortage! Run! Get it while you can!"
Nintendo said a long time ago that the Japanese DS launch shipment was going to be 300,000 at launch day, and build up to 1 million available before the end of the calendar year.
The American DS launch is supposed to be very similar, but (last I checked) NOA hasn't given out any numbers. They'll "have as many units available, as quick as they can".
The "worldwide" DS number is expected to be 4.5 million by next March (the end of the fiscal year).
Then two million Japanese people preordered the DS, even though only half of those people are going to be able to get one anytime this year, so Nintendo said "Oh snap. This thing's really popular. What can we do to make more units available?" Then they announced that they were outsourcing more stuff to China, which should help them make even more units, but they don't know exactly how many.