Well it depends on how blurry you mean. I've noticed that above speeds that I couldn't hand hold steady the images can be less sharp with IS than without. This effect seems to be random. On the 24-105 at say 50mm I would always have to use IS up to about 40th. Between 40th and 125th I tend to shoot some IS on, some off. Again talking 50mm focal, I can see the difference in sharpness of an image taken at 200th when hand held or from a (very) rigid tripod.
Sometimes I've even got a sharper image hand held with 50 1.4 at 30th than an IS lens - but again it's very random.
It's a strange thing is camera shake, really quite difficult to eliminate altogether, as the proud owners of the D800 are finding out
Are the effects of shake / movement exaggerated in a dense ( v high res) chip ? Maybe some of the tech bods can shed some light.......