Nintendo still struggles to put enough Wii consoles into store shelves. Amazon.com seldom has a Wii in Stock. They even put now a new policy in place that allows you to just buy 3 Wii consoles per calendar month in order to give customers a more even chance on buying a Wii console.
The Wii stock situation is supposed to improve dramatically in time for the holiday shopping season. At least this is what Nintendo's Reggie Fils-Aime told the San Jose Mercury News in a video interview. He still warns though that the demand is so high that the "unprecedented" Wii supply is still going to sell out quickly.
More details in the Video interview on Mercury News. Via Kotaku.
The Wii inventory problem is not an issue in the UK or Germany. In both countries you just can hit the Amazon store and order it anytime: Wii on Amazon UK, Wii on Amazon Germany.