Hello Dave,
The Canon instructions for my 70-200 f/4 IS also say to turn off IS when tripod-mounted. As I mentioned in another thread, it appears to me that Canon is assuming that camera shake is extremely low or non-existent with a tripod mount configuration. If that were true, it might be better to turn IS off to avoid the unlikely possibility that the gyro feedback system decides to change the active lens element angle at precisely the time when the shutter is open. A more technical explanation is that electronic (or in this case, electro-optical) feedback systems constantly "hunt" around the ideal setting (in this case the active lens element position that results in a zero error (feedback) signal at a time when the gyros register zero acceleration). Turning IS off freezes the active lens element in a fixed position. Theoretically this would provide a rock-solid image provided that the camera is absolutely fixed relative to the scene.
In reality, though, the image can be quite unstable with a tripod-mounted long lens. In my experience, camera shake with my 70-200 at 200 mm with 1.4x converter on my 40D (200 x 1.4 x 1.6 = 448 mm) is on average high enough to benefit from the IS system being active. I conclude that any blur introduced by the IS system is much smaller than the tripod-mount camera shake due to wind, mirror locked-up shutter release, etc. Thus, I ignore what seems to me to be a misguided instruction in the IS manual. It could be, as you suggest, that the IS systems in older Canon lenses are not as stable, and that Canon has simply carried the old text forward to the new manuals.
