I popped a new cpu into the last slot on my motherboard.
It broke ubuntu at first... but I boot into "rescue mode" and entered the command aticonfig --adapter=all --initial twice and it found all of the video cards (there were 5 now listed, instead of 4).
After rebooting I can only mine with the original 4 cards.
aticonfig can see the 5th card, but poclbm can't.
When I reinstalled my card I had some odd behavior too. I reinstalled the fglrx package so it would recompile the driver, redid the X config like you did, then ran CLinfo to make sure it was seeing all cards. That cleared it up.