I would say that if you have a backpack with a padded laptop compartment, you will be ok.
There have been times where I had a backpack on and I sat down on the DC Metro train with full weight and not damaged my laptop. I think the fact that it was a regular backpack and not one designed for laptop protection, you will be fine with using it in a backpack.
My favorite ones right now are the Waterfield backpacks.
I do have one backpack that "says" it supports a laptop, but the padding is very weak and minimal. I bought it anyway because it is a collectors edition Nintendo retro NES backpack.