Not that big of a deal. Alienware is mostly marketing hype-- they overcharge for what you get. They are basically charging you a major premium for the name brand and a brightly colored case with neon lights in it. Dell, whose bread and butter business is selling cheap computers on the enterprise market along with service contracts, has been trying to get in to the high priced gaming rig market (with high mark ups) for a while with their XPS line. They haven't had that much luck though, and the XPS computers are pretty much speced exactly the same as their high end desktops, which you can buy for a bit cheaper.
Also, the recent rumors were that Dell was majorly POed at Intel for teaming up with Apple for the CoreDuo launch and letting Apple out of all of the branding restrictions that Intel subjects the rest of its customers to. They have contracts with Intel so while they'd like to sell systems with AMD chips in them (if for no other reason than to flip Intel the bird), from a business standpoint it might not make sense. But having another division that allready has pre-existing contracts with AMD means they can do this and not technically violate any deals that they already have. But Michael Dell still gets to let Intel know that he isn't happy.