According to reports, Nintendo sold approximately 1.5 million DS systems in December alone in Japan and early numbers suggest that they sold another 400,000 over the New Year Holidays, leaving the systems currently in a “completely sold out” status.
Nintendo has actually issued an apology (Japanese) on its website, which has been translated as follow:
We heartily thank those who bestow their patronage on our humble company.
Over the New Year season, demand for our portable game system “Nintendo DS” has vastly exceeded our predictions. It has gone out of stock, which we know has caused a great deal of trouble to our honored customers and retailers. For this we apologize from our hearts.
We will begin to ship our product by air mail, as this will help it get to retailers a bit faster. We are shipping product every week, but as supplies will not be fully replenished until the end of January, we humbly supplicate ourselves before you and beg your favor.
[...] We know that Xbox 360 faces a severe shortage in US, and the same thing happened for the Nintendo DS in Japan. However, Nintendo reaction towards the problem were different. [...]