I decided to put CentOS 6 on my iMac. It was running in dual-boot mode with OSX and CentOS 5. Installing through the network (from a NFS share) was really easy and no bug encountered but at the end of the install, when it asked me to reboot, nothing : after having selected the Linux partition in the rEfit boot manager screen, nothing. hmm ....

I restarted the install process to see if at least anaconda tried to install grub on the first sector of the /boot partition and not in the MBR but that was correctly seen and chosen by anaconda . So the issue was somewhere else. I had a /boot ext3 partition (on /dev/sda3) while /dev/sda4 is the VolumeGroup in which I had defined my Logical Volumes. There was a big rewrite in Anaconda for the storage part and el6/CentOS 6 suffers from one bug found on the upstream bugzilla when having to deal with Apple computers *and* using rEfit at the same time : https://bugzilla.redhat.com/show_bug.cgi?id=505817

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