Pay attention to the Sunday circular! If they advertise (front page or otherwise), that retailer is absolutely getting some that week. They'll usually go on sale on that Sunday, but not always. They might come in the middle of the week. Trust me, I worked at Targer not too long ago. I quit because it sucked, but believe me. Also, from my experience, Target only gets shipments above 15 whereas Toys R US and Walmart will take whatever Nintendo can spare them which can be as low as 6 or 7 consoles. That said, Target doesn't seem to get Wii shipments as frequently, but when they do, they're typically larger than other retailers. Bear in mind that I'm only speaking of stores in my area so it might be different elsewhere though I assume it's all the same.
Still, Nintendo apparently ramped up Wii production in April so I would expect Wii to be more available soon.