How do you know that it will be proportional. Also you dont know when this "poportional reduction" will take place
Its a matter of strategy.
Nintendo has less reasons to reduce prices than Sony. They are already enjoying lots of sales from low price as well as profits from hardware. It would be stupid if they did this. They will reduce profits unnecessarilly.
Sony cares more about increasing its userbase right now than Nintendo, and there is more room for price drops because unlike Wii's harware it is not standardized yet. It is new, it is passing through more production readjustments and has more components which overally will decrease the hardware's price much more while their cost of production falls. Lastly Sony has always been making price drops even though they generated losses during the initial stages, as part of their strategy.
The Wii's price is tied to its demand and as of right now the PS3 or 360 has done very little to lessen that demand. But, if the PS3 sales begins grow at the expense of Wii sales then Nintendo will have no choice but to reduce price.
While the demand in japan has fallen off some, demand in Europe and the US remains high, which means what Nintendo has the luxury of shifting more units to the US and Europe to keep production at close to max capacity.
Nintendo had to work hard to increase supply to deal with the initial demand of the Wii. Nintendo's primary drive is to maintain the demand to eat up that supply. Its not going to try to maintain the current profit margin on the Wii at the expense of growing inventory.