Perhaps others can confirm or correct me, but I think the 70-200 has a newer generation of the IS
system, which obviously (as luminous landscape states?) can be mounted on a tripod with IS
switched on. The 100-400 could get confused if IS
is switched on when the camera is completely still, causing IS
lens-group to shake. I have not noticed this yet, but people with the 100-400 have been advised (informally, I don't know what Canon has said) to switch off the IS
when taking shots from a tripod. Which isn't a problem really, since you don't need IS
on a tripod.
[Edited 2004-11-06 21:09:26]