Why assume the PS3 price cut is coming in Spring (they've repeatedly denied it, while only a few unlikely rumors back it up)?
Second, I very much doubt the cut if it comes in spring will be $100 (if it is, it basically means no further cut for all 2009). So if it's $50, which if there is one at all I think it will be, that brings PS3 to 349, and I dont think that will have any big effect.
I dont expect MS to cut again anytime soon. Come on people, they just got done cutting in October. It will be fall 09 at the earliest (and I think the next MS cut may be small, especially on the Arcade which is already very cheap). I could see the next MS cut being 30/40/50 Arcade/Pro/Elite.
MS wont react unless they really need to, and a 50 cut to PS3 wont make them need to.