OK I upgraded to Linux 2.6.6-rc1 and replaced /sbin/hotplug with this script:
http://dev.gentoo.org/~wmertens/automount.hotplug
(There is also a version for Linux 2.4 on that website.)
It does what I want, after a few modifications to mount accesible to all users, and to create an icon on my KDE desktop. But it only works sometimes

- sometimes the /sbin/hotplug script is not called at all by the kernel when my USB Mass Storage Device is inserted and removed. This happens for example if I have not used the computer for a few hours. Anyone know what the problem is?
Thanks,
Mark.
--- edit ---
I have noticed that when the script is not called the /dev/sd* device file is not created either, so it seems to be some problem with the kernel detection. Usually works the first time, but not always afterwards... has anyone else seen this...?