There's an interview with Steve Ballmer on Engadget. He says they may have a next-gen drive on the X360 somehow. He won't commit to a Mark II X360 but he said it would be "wrong-minded" to think they'd go through a whole generation without some capability. He also said Sony has picked a "non-standard" drive which may go the way of Beta.
He thinks they will have market leadership, maybe over 50%, even though Sony will "outperform" MS in Japan.
Well on the one hand, you can't promise an upgraded model with a blue-laser drive, because it would splinter the X360 installed base and maybe make some prospective purchasers hold off. On the other hand, they can't be seen as offering something lesser. But they think launching first is crucial so they may have accepted the possibility that their product will be seen as inferior for only having a DVD drive.
Later in the interview, he says some publishers have told him that within two years, they will need "massively more storage." So they will figure out some way to deliver that but for "this Holiday" they're going to concentrate on one configuration. Again, he can't say there will be other configurations but he's got to leave the impression that there will be some kind of upgraded SKU later. Robbie Bach is there and chimes in from time to time so it's not a case of Ballmer going off message.