I think a percieved loss of value would be worse for Microsoft and Sony. I think it's also a reason why Nintendo are wary of cutting the price of the Wii-in mass market terms, it's been cheaply priced since launch, so any cut making it even cheaper could be read as the Wii being less valuable and therefore less desirable.
That being said, I think it's unlikely we'll see further significant price cuts from Sony or Microsoft for a year at least. Perhaps they'll bundle their motion controls in as standard, to try and increase the value of the system. Bundling is a strategy I expect Nintendo to try before they cut the price of the Wii. I think a NSMB Wii bundle is most likely, but other than that, it's difficult to say. Wii Sports Resort (with M+) and Wii Fit Plus (balance board) bundles are also likely sometime next year.