The issues with price cuts is more complex than just doing calculations of how many units they will temporarily increase their sales with how the price difference of the original cost and see if the outcome is worth the potential loss in revenue for Nintendo with a price cut.
The reason being is that markets aren't static, they are ever changing and number of variables can change market trends. The price decreases for Xbox 360 and PS3 will have an effect on the market, and it will directly effect the perception of the Wii. Whether those perceptions are enough to change market trends is yet to be seen...but if Nintendo is caught being reactionary after a markets perception changed it is too late...that perception shift has already began to take place.
What I mean is right now Nintendo Wii is perceived to be a very good value and a great system for casual gamers, families and the like. Nintendo doesn't even want to begin jeopardizing that by allowing Xbox 360 or PS3 become an attractive purchase. Even casuals and non gamers can look at the graphics, power, and all the other 'extras' that come with the other systems and see it as a superior system. They still may not enjoy the games on there, but they may see Nintendo Wii as an overpriced system and reject it as well.
A price cut in a highly competitive market is sometimes needed to maintain growth, and yes the initial sales spike is only temporary, but IF a shift in perception happens and perceived value of the Wii in the market shifts that is more dangerous for Nintendo...and after being last a generation ago, being first place this generation for only 3-4 years but fizzling out would still leave consumers with a bad taste in their mouth. And once again question if Nintendo truly has what it takes to compete in this market.
I don't know if a price cut will happen, or even if it is needed...and I certainly don't know what any cut would look like. But I do know it is more complex issue than just number crunching, and Nintendo needs to weigh all the scenarios before making their decision this holiday season.