If nintendos estimates are correct in the number of Wii's available at launch, then there shouldn't be much problem getting one at or around launch time anyways.
Probably will have to wait for a long time in an even longer (and colder) line for those... but at least you will walk away with something other than a runny nose.
Last estimates I heard were 1 million units available in North America on launch day and 4 million units worldwide before the end of the year (with most sent to Nor. America). If all of those sell out in the first hour or so they're available, then Nintendo should easily win this generation's console war. I have a feeling that you'll be able to buy a Wii on launch day without waiting in long lines and without a preorder, if not, then I'm willing to wait a week or two for the next shipment to come in.
Just to give a point of reference, Sony is estimating 400,000 on launch day and the 360 had only 1.5 million available worldwide by the end of the first year.