I carry my iBook to and from University (about 5 miles, okey, I normally only walk one way and get driven the other) and drag it around campus all day, always just put to sleep in its tucano second skin and tossed in a backpack. No problems, whatsoever...
I even managed to slip and fall
twice last week - it was bl**dy cold and the ice was not removed from the pavements early in the morning...
...the first time I actually landed on my back (after first hitting the ground with my left knee) and the backpack took all my weight, the second time I had the backpack over just one sholder and (except for my poor left knee which again took a beating) the backpack was the thing that hit the ground the hardest... the iBook still woke up when I got indoors, both times, and works like a charm...