Camera shake occurs at all shutter speeds. Whether or not it is optically evident in your photos is another question.
For example, imagine a camera duct-taped to a vibrator (yes, i'm being serious here, you pervs.) You trip the shutter twice, one frame at 1/30 shutter speed and another at 1/8000 shutter speed. Camera shake in both shots was the same, regardless of the shutter speed (or even presence of IS), right?
That said, IS is useful at all shutter speeds, in fact, Nikon sells a 200mm f/2 lens that has VR (Nikon calls it VR; Canon calls it IS). We shouldn't forget the famous 70-200mm f/2.8 VR.