It all depends on how many systems MS supplies. They will probably hold back to sell out of the older 360 stock. Total 360 sales (older units - 50 (plus) slims) will sell around PS3 slim number (older ps3s with pricedrop (plus) slims at -$100)
It also depends on what Sony does. If they have a price drop at their show, there is a good chance momentum will stay put (not saying there will not be any increase, but not as great as it should have been)
edit: fucking (plus) characters not working