I've noticed a lot of chat on the forums recently about IS - Image Stabilisation - so when the latest issue of appeared in my letterbox with an article about IS, I took it as a sign. 
Here's a short summary of some of the key facts in the article (from the Sep '03 issue).
Image Stabilisation is a Canon solution to the problem of camera shake. It uses two gyroscopic sensors that sense the pitch and yaw (rockin' and rollin' ) of camera shake that so the image on the sensor/film does not change.
...when you press the shutter button, if IS is on:
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The set of stabiliser lenses is unlocked from its central position.
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The gyros sensors start movement detection.
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Movement data is analysed and compensating instructions calculated.
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Compensation instructions are sent to the stabiliser lens group, which adjusts to counteract the camera shake.
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The sense-analyse-adjust sequence is repeated continuously until after the shutter button is fully depressed and the picture is taken.
It takes about 1 second for the system to become fully effective (except on the EF 70-200 L IS which is twice as fast) and this is all done without affecting the optical performance of the lens.