i also have a mobility x1400.
after a few tries i got it working.
on a fresh install of gutsy, after i installed and configured the ati driver. i checked my xorg.conf and found composite = 0 and no AIGLX section. when i added the AIGLX section it screwed the driver up.
try setting composite to 1 and removing the AIGLX section. and you said you have the whitelisting done, so that should be it.
this worked for me. hope it helps.