I found the answer and could download the good driver there :

http://dl.dropbox.com/u/983756/sis671_10.x.tar.gz

It even works better than it did on Karmic as I have now a 1280*800 screen.
My only remaining problem with the Lucid upgrade is that on the "grub2" menu,
the second choice "recovery" does not work, which means I cannot currently
recover from a severe crash but I hope it will improve as Lucid is more and more
used.