I've been to, down into, and through the Grand Canyon a few times, and you'll definitely need wide angle capability. The scale is truly awesome. Don't forget about the stitch-assist mode on the S70 (I use one myself as a backup to a Rebel XT) for scenes that are too wide for a single frame. I turn mine vertically and use the "top to bottom" stitch mode to get a stitched image that's 3072 pixels high instead of the 2304 you'd get if panning with the camera horizontal. Autostitch produces much better stitches than Canon's software, quickly and easily. Only caveat is stitch assist and Autostitch only work with jpegs, not raw.
You could probably get by without the 70-200 lens since the 28-135 is 45-216 "equivalent". I've hardly ever used telephoto in the canyon, but I suppose your choice of subjects may be different.
If you're rafting, you'll need a relaible way to keep the equipment dry. Splashes are inevitable, and submersion not unlikely.
Enjoy the ride!