^ I thought so too, but I found out that a few P55 boards don't support SLI, if Alienware didn't bother paying premium for SLI capability (this is an assumption, OEMs wouldn't want to pay what the customer didn't specifically ask). Like the P55A-UD3R and EVGA P55 LE for example. But then again, you're probably right in the sense that Alienware is pretty high-end and would have used an SLI-ready board.
I looked it up and it should support SLI as that motherboard uses an i7 900 chip, despite the fact that the only people which have that motherboard seem to be using HD 5870 CFX.