Continuing investigation, I've tried to start with expert26 and at the "detect CDRom" step, I disabled 'sata_sil'. This allowed me to go a little further to the Load DEBconf step, but again, the install script got stuck while "Loading module 'sata_sil' ..."
I've found a lot of topics regarding linux 2.6 and Sil 3112A S-ATA controller. It seems there is known problem with this hardware and that some got fixed with version 2.6.12.
What version is Pluto 188.8.131.52 based on ?