I don't think MS's plans really matter to Sony in this instance. The people who are waiting for a price drop to "Play Beyond" probably don't care too much what the price of a X360 is. Obviously there is some overlap in the two markets and the X360 would retain a clear advantage on at-purchase decision makers. But beyond that I think a Sony price cut is aimed at people waiting to buy a Sony console...not just any console.
The reason is that at this point everyone knows the 360 is cheaper so that advantage is already factored into the situation. A reduction in the price won't change that perception so I don't see how it would change the purchase rate. And that is the important distinction to make here, I'm not saying this is a bad plan by MS, it actually is a good move. But the price cuts from each company have entirely different goals. Sony simply wants to increase sales volume in the hopes of increasing software revenue and in general it's all about volume for them at the moment. MS's move would also be about volume to an extent, but the real goal is in protecting and gaining market share.
That's my take on the situation at least.