I keep tent, poles, stakes in their stuff sack in one of the outside pockets of my pack, so it's available to set up in pouring rain without having to open my pack. If I were to carry the tent inside my pack (which I may start doing since I recently acquired a cuben fiber tent), I would keep it right on top and separated with a waterproof barrier, probably a pack liner, from everything else inside, since said tent often gets packed wet.

On the lightness topic, I wholeheartedly agree with Aimless and Gershon!