Bjørn Rørslett mentioned in his
review of the Nikon 70-200/2.8 VR lens that the VR settings affects bokeh at shorter focal lengths. I haven't confirmed if there are differences with the Canon lens, since I've been using it indoors and shooting at 1/60 second (with lots of flash).
I've some examples of background blur from a 70-200/2.8 IS with the IS on below:
