I've done some research into Canon literature about their Image Stabilization for EOS lenses. I am going to give credit for my endeavor to diverhank , who posted a link to the following article, which discusses using IS on a tripod.
http://cpn.canon-europe.com/content/education/technical/image_stabilization_lenses.do
For some lenses, it could be best turn off IS when the camera/lens combo is mounted on a trippd. I found more stuff, though.
http://cpn.canon-europe.com/content/education/infobank/lenses/image_stabilisation.do
On page 4 of the above article, you will find a chart that list lenses that can and cannot detect when it is mounted on a tripod. on page 3, it describes the behavior of many [now discontinued] cameras with IS systems, such as when and how much, or how little, the image can shift in the viewfinder.
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Here is an interesting link about USM technology.
http://cpn.canon-europe.com/content/education/technical/usmlens_technology.do
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