you may have gotten libxcb-1.8.1-1.fc17.x86_64 from the updates-testing repo before it was disabled (or sometimes at this stage of development the spins have packages that haven't yet made it into the stable repo), either way the i686 and x86_64 packages have to be the same version, you might try
Code:
yum distro-sync; yum install libX11-devel.i686
if the above is true, this will downgrade your libxcb to version 1.8-2 to match the i686 package in the repo