I have an SLI system and ( I believe my motherboard is damaged ) most of the time the second card isn't "present" so sometimes when I boot, there are 2 cards, sometimes one. Now, when this second card magically appears, xorg can't figure out which one is the primary device ( I'm fairly sure the first one would be a good default ) and I get "no screens found" in the logs and X won't start. Took me a little while to figure that out and fixed it with a Busid line in xorg.conf.
Probably would have fixed it faster if I'd actually looked at the logs straight away and I'm fortunate to have multiple computers ( quick google search led me to the answer ). I'm guessing the problem arose from the second card not being present during the installation.. so the hardware detection still has some ( minor but significant ) gaps.