It's not that demand is low. I think the Wii will be flying off the shelve. It's that supply will be high. The Wii is not made up of state-of-the-art technology. Reports are putting yields of PS3 chips at below 30% (Meaning that for every good, useable chip produced, two duds are produced that can't be used in a PS3) and the blue diodes for the Blu-Ray drive are in equally short supply. The PS3 is filled with bleeding edge technology that's never been made before. The Wii, on the other hand, is filled with technology that's been around at least a couple of years so manufacturing won't be a problem. Sony is allocating just 400,000 units to the entire U.S. and 100,000 to Japan this year. Nintendo will be putting millions of Wii's on store shelves in the same time period (they are planning to ship 6 million by the end of March).
It'll be very popular, but much MUCH easier to find than the PS3.