It's only been ANNOUNCED as an add-on, but I'm 99% positive that shortly after the external drive comes out there will be a "refreshed" 360 with the new drive built-in, if only to make the 360 more attractive compared to the then-available PS3's built-in BD drive.

Of course they wouldn't announce such a thing in advance, since it would potentially impact immediate 360 sales (better to assure everyone that they can "upgrade later").