@Killerman (and TWRoo): TWRoos reasoning is only a part of the truth. Nintendo isn't much behind in going to smaller chipsets, but instead of looking why Nintendo isn't going to smaller die, incentives of Sony and M$ should be observed.
M$ had an incentive to get 360 to run cooler due to RRoD issues and since it is sold at a loss, cutting the cost is important.
PS3 is sold at a loss so Sony has a need to cut the cost too.
Wii is sold at a profit and it doesn't have a hardware failure similar to RRoD, so there's no forcing need to get the cost down. After all, i'd recall GC having the same CPU and GPU through its life.
Due to the difference in number/size of processors, going to smaller size doesn't pay off with Wii in similar fashion it does with PS360, so it's much better to wait until the smaller sized dies are noticeably cheaper or go by your possible contract that has the option to shrink in size after a certain period of time.