I have often wondered about this same question. I
have a Powerbook G4 and I have been carrying it in a
backpack every day for 2 years and never had a single
problem. Am I really just lucky or is it kind of over
dramatic to say that you should not carry around your
laptop in a backpack?
I also do not understand the advice against backpacks. I have been carrying my PowerBook in a backpack daily for
four years and it's running so well I've put off the MacBook Pro upgrade.
I sometimes walk for relatively long distances (compared to most people) with my laptop. I picked a backpack because when the laptop was in a shoulder bag, the middle of the bag (and the center of the laptop) would repetitively impact my hip as I walked, a force focused on the middle of the laptop. It was also an unbalanced load on the body. That seemed like a recipe for damage to both laptop and body.
In a backpack, the laptop's contact area is evenly distributed and dissipated across the entire back. There is potentially more oscillation in the vertical direction, aligned with the length of the laptop, but I see that as less stressful on the laptop.
There is still some repetitive impact between the laptop and the back, but if you use a backpack you should only use one with thick padding on the back. Mine has two vertical pads, so not only is there sufficient shock absorption, but there is a gap in the middle to vent body heat. It has to be a good backpack - the difference in comfort and protection between a good backpack and a minimal one is dramatic.
My PowerBook is now 7 months past the expiration of its 3-year AppleCare coverage and it still runs like new.
It is possible that what people are really concerned about is compression. In that sense, a laptop bag has no advantage over a backpack. If you overpack either, the laptop is compressed. If either bag is not full, the laptop is not under pressure. In fact, I would probably pick a backpack, because the laptop is furthest in and closest to the body, which means the other items in the backpack (heavy books, etc) are farther out in the pack, pulling it down and away from the body, actually drawing the force away from the laptop and not pressing on it.
The one danger with a heavy backpack is putting the pack on a table or chair on its back, with the laptop underneath everything. Don't do that - instead, stand the backpack on its bottom, which stands the laptop on its end so that it takes no pressure. Sometimes a filled backpack will topple forward, but that's perfect, because the laptop ends up on top of the pile, again avoiding the pressure.
I can see favoring a laptop bag if 90% of your laptop transportation is the 200 feet between the parking lot and the office building with the bag held in the hand, so there is not much opportunity for lateral impacts concentrated on the center of the laptop as there is when the bag is on the shoulder strap.