The reason this is possible is because the express card interface is a PCIe interface with a differenty shaped conenctor, but it is the same electricaly and to the OS and software. This means that all the ASUS box need to do is provide an enlosure, power and cooling. There is no need for any complex bridging chips because all the electrical protocols are the same.
Personaly I think alienware stuff is to expensive for what you get, and your envisaged alienware laptop would probably come out quite large and heavy anyway once they had included a fast CPU with a massive heatsink and a pair of RAID 0 discs. This would be much better as an upgrade for any laptop.
That would mean extra fat on your ultra portable laptop for a 17" display and the large battery needed to drive it. I would say it would be better to stick with a small 12" display, and have the option to hook up to an external 24" or larger dispay when necessary.
In any case, it would not be possible useing a standard express card connector, and the big selling point of this idea is that both the laptop and the graphics card are completely standard retail kit at commodity prices.