All of the 70-200 IS lenses have "second generation" IS. And "tripod mode" is NOT the same as an "off switch". When on a tripod, the IS mechanism will still activate (you can hear the whirring sound) and vibrations transmitted through the tripod, such as from wind gusts or the camera being bumped, are significantly reduced. There is a definite benefit to leaving IS on, even when mounted to a tripod. You can easily see the difference through the viewfinder, and the difference is also apparent in the images.
If the tripod is not sufficiently stable, obviously IS will be a benefit. However, if the tripod is stable enough for the lens to go into "tripod mode" then the IS shuts off. On teh 70-200, "tripod mode" = automatic shitoff.
This is different than the IS in the super telephotos (300, 400, 500, 600)where "tripod mode" means that minor vibrations from mirror slap are reduced by IS even when the lens is on a stable platform. This is not the case with the 70-200.
I was under the same impression so I emailed Chuck Westfall, who is a Canon spokesperson, and he sent me the above content in an email explaining the various incarnations of IS.