Yes Nintendo is planning to ship 4 Million units worldwide (Japan, Europe, North America) by the end of 2006. Planned is 1 Million at launch for North America (US and Canada). With 1 Million left to be trickled down (divided up amonth the other countries) shipments till the Holiday Seasons.
14 Million people watched Heroes on it's debut in the US. 1 Million shipments seems like it's nothing compared to those numbers. Now imagine Nintendo starts advertising ;x . The 1 Million units are divided up between US and Canada, so let's say Canada has population of battalion of mooses and get's 300,000+/- Wii's. That would leave 700,000+/- for the US at launch I guess?
In store Pre-orders are sold out from Gamestop/ebgames and Nintendo World Store. Nintendo hasn't even started putting advertisements on TV yet and in store pre-orders are sold out. If Wii becomes a hot item this Christmas season chances are it can be sold out. Of course not saying that you won't get one but anything can happen.
*Edit* Bolded the 1 Million, Also the numbers are taken from the Nintendo Press Conference held in Sept. 2006. Official words and not some analysts that don't work for Nintendo.