Well, it supposed to help get sharper images when handheld, even at high speeds.
But with a tripod, where movement is miniscule, it is recommended that you turn off IS, because with a camera on a tripod, IS may activate and move during the exposure and will cause an image that is not optimally sharp.
From Canon USA spokesman:
"The IS mechanism operates by correcting shake. When there is no shake, or when the level of shake is below the threshold of the system's detection capability, use of the IS feature may actually *add* unwanted blur to the photograph, therefore you should shut it off in this situation.
When there's not enough motion for the IS system to detect, the result can sometimes be a sort of electronic 'feedback loop,' somewhat analogous to the ringing noise of an audio feedback loop we're all familiar with."
.