Si ça peut aider, voilà ce que j'ai trouvé par ailleurs :
Long story short : to have CentOS 6 running on your iMac (if using refit as the EFI boot manager) :
install CentOS 6 as usual (check that grub will be installed on the first sector of /boot and not in the MBR , normally correctly seen/proposed by Anaconda)
on the first reboot, enter the rEFIt shell and launch 'gptsync' (it will say that it has to 'sync' the gpt, accept the sync)
select now the Linux partition : it will fail with a black screen
power down the iMac and start it up : select Linux in the refit boot manager and enjoy your CentOS 6 installation on the iMac