If you have used the S1iS's movie mode, then you are probably aware that it's pretty good: 640x480 resolution at 15 or 30 frames per second. You've probably used the zoom and noticed that the exposure is automatically adjusted.
There are a couple of other functions available, which you might not have noticed tucked away on p. 56 of the camera user guide. One turns off auto exposure, the other locks the focus. To try them out, turn on the camera, and then set it to Movie mode.
Change the exposure. Press the Light Metering [o] button, the second button below the movie button (with the red dot). Notice the bar appear in the lower part of the viewfinder. Press the left and right buttons of the 4-way controller. Point the camera at the window, and notice that the image gets bright and darker. This lets you change the exposure away from that selected automatically by the camera.
Lock the focus. Press the MF button (found on the barrel of the lens). Notice that the AFL icon appears. The focus is now locked, and won't change as you move the camera about.
There is one other function documented by Canon, which I think does not work. Page 163 indicates that Movie mode works with 2nd-curtain flash sync, but the flash doesn't work with Movie mode.