Nintendo is selling an amazing amount of Wii systems worldwide every month, and they are making money on each one. Since they are reaching previously untapped markets, they do not know exactly how much demand there is for the console. If I remember the interview comments correctly, a Nintendo rep said that they had already ramped up production four times since the initial launch.
The Wii is popular, plain and simple. I feel for those who are looking for one and cannot find it, but there have been plenty of opportunities available to purchase one in the last two years. There may be a shortage, but I have seen no convincing evidence that it is artificial.
Nerdius, my guess is that Nintendo asks stores to hold them until certain dates because it is easier for retailers to sell 20 Wiis one day a month than five Wiis four days a month (or whatever the numbers may be). It's also easier on the shoppers. They know what day to visit the store, and they stand a better chance of getting one without throwing elbows around.