Hearing that kubuntu jaunty was out, I upgraded two machines today.
So... I recompiled the xorg packages from intrepid (i.e. X.org 1.5.2) so that they can be used on jaunty. Long and painful, but it solved the problem. Proof that the problem is really X.org 1.6.0.
In case anyone is interested: http://www.davidfaure.fr/2009/xorg_debs.tar.bz2
Quick instructions: download, unpack, then
If you see apt-get -f install removing a bunch of video drivers, that's fine; I didn't recompile them all, only a few.