My local Target (Sherwood, Ore) is going to be passing out tickets at 7:00 am for the 8:00 am opening and said that they are welcoming campers; hopefully that means smores and hot cocoa, but I doubt it. If you want to keep track of Target's supply of Wiis, you can use this inventory tracker...just replace the last part of the URL with your zip code.
Best Buy is the only company promising a minimum number of Wiis at each store: 20. Otherwise, it's hit and miss. I don't think you'll need to sleep overnight in most locations, but I would recommend a 5:00 am start. Good luck...it's worth it!