Amazon just informed via email that the Nintendo Wii will be available for purchase starting Sunday morning.

Amazon will be limiting purchases to one Wii per household and anticipates that they will sell through their inventory very quickly as they received 100x more Wii email sign-ups than consoles waiting in their warehouses.

I have no idea what exact time Sunday morning means and in which time zone. Better check the Amazon Wii page as often as possible.
Update:
it is 3:30 PCT and Amazon has not yet started to offer the Nintendo Wii. Tons of would be customers are refreshing the page every second. There is no word still at what exact time Amazon.com will start offer the Wii. In the meantime Microcenter is offering the Wii online.

Toys"R"Us is supposed to start offering the Nintendo Wii starting Sunday at midnight.
More Nintendo Wii news.