I"m not sure 360 sales WERE disappointing. They sold more this holiday than the last 2 (that hasn't changed, right?). They just look weak compared to the PS3, which is benfiting from a $100-200 price cut (depending on how you look at it). Since MS can't give their console away in Japan, and wasn't able to match the Sony price cuts, this shouldn't be a big surprise.
MS will almost certainly cut the price of the 360 to under $300 before E3 (and I think before May). If they were smart, would announce an even bigger cut for Europe, and consider marketing an inexpensive ($99) BluRay player add on.