As far as MS' business goals are right now, it may be more important to maintain profitability than it would be to compete on sales numbers with Sony. Perhaps MS doesn't care if Sony outsells them for a few more months, as long as the race is close, which it inevitably will be for a long time.
Perhaps. I think Sony is pretty happy with sales right now and they're probably not particularly interested in hurting their bottom line with another expensive price cut.