It is not working even if I try ubuntu before. Moreover, I am quite sure to set the nomodeset correctly and after I choose Install Ubuntu from the menu. Is there something special to do after ?

I believe that when the problem occurs, the installer checks the disks status and the existing partitions. So, maybe there is a problem with my partition table but I don't know what. I have a partition for Mac OS X, a swap space of the old version of ubuntu (11.10) and a free space formatted in Ext4 where I want to install ubuntu 12.04. I also have a SSD.

I found on the french ubuntu forum people with exactly the same problem : http://forum.ubuntu-fr.org/viewtopic.php?id=897031