OK, I still don't understand what exactly went wrong, but my system boot nicely now. I think the problem was here :
Trying to resume from /dev/hda3 No suspend signature on swap, not resuming. Creating root device. Mounting root filesystem. mount: error mounting /dev/root on /sysroot as auto: No such device
^^^^ This is because, when I ran mkinitrd, /etc/fstab was reading: /dev/hda1 / auto defaults 1 1 instead of : /dev/hda1 / ext3 defaults 1 1
I am not really sure this was the problem, and can only suppose that the support for ext3 was missing in the initrd.img (ext3 as a module, they are crazy ;-), but I re-ran mkinitrd after editing fstab and voila.
Thanks to all of you for your answers !