Hey, thanks for the link.
I tried that but I have some troubles working in that mode-can't nano or stuff. Maybe I did something wrong.
Anyway, I loaded my old live dvd of SL4 and used
sudo chroot /media/disk/
and everything is working again. Nice
Ok, I'm still not in the sudoers file, obviously, but that will soon change
Have a nice day. (And yet, maybe that update shouldn't have messed up with my permissions, right?)