The effect is the same but the process is different. Alienware's ALX can use any graphix cards whereas the nVidia system uses two of their cards. If you take a look at the new nVidia cards, they have a small connector at the top. That's for a jumper board that connects the two cards. Here's an article on it for the curious: http://www.nvidia.com/page/sli.html
Its even got a little video to entertain you.
Neither of these are likely to turn up in a notebook any time soon, the Alienware box is half the size of most notebooks and costs twice as much. The nVidia solution is only for their latest cards and those are double slot monsters as it is. Both solutions suck more power and generate more heat than two of the hottest Prescotts.
But, its been done and there will be pressure to do it in a notebook someday. But I fear that someday is a long ways off. More likely is something with dual core gpus. Those are probably two years out.