I was looking at a tech article on the 70-200 f2.8 II on Canon's camera museum website here:
http://www.canon.com/camera-museum/tech/report/2010/08/
and noticed these two comments regarding IS:
*1 Image stabilization generally reaches its effective limit at a shutter speed of about 1/[focal length] seconds.
*2 Image stabilization is most effective at shutter speeds between 1/25 of a second and 1 second.
I found the first comment interesting because it reveals Canon's technical thinking on IS, which is significant because they invented the technology or at least brought it to market first. The issue of IS or no IS has come up several times at BPN with the conclusion that if it doesn't provide benefit but costs nothing, leave it on anyway. Costs could be lower battery life, wear and tear on the IS/VR system, and Roger has mentioned an issue with the 300/4 at high SS, so costs are potentially low. But should we really be heeding Canon and turning IS off at 1/300s for a 300mm lens?