I ride to school with my laptop in my backpack routinely. It is a constant source of stress for me. I continually worry about going down with that big ass thing on my back. My concern is, even with a jacket on, the laptop will be solid enough to fracture my spine if I land wrong. Nightmare situation. Most likely this isn't something I should focus on, but it is something I think about almost every day. Plus it is considerable extra weight.
Oh, and I have that Swiss Gear backpack. It's a monster. the thing is gigantic. Almost too big, but it will pad your laptop nicely. I now use a Jansport I picked up at Sam's Club. It's not made for laptops, but mine fits nicely in a separate compartment that seems to keep it relatively safe. Like has been said before, if you go down, the laptop will probably be the least of your concerns. Just be aware that if you DO go down, you WILL have a broken laptop unless you have incredible luck (in which case you should go buy a lottery ticket ASAP).
Sorry I don't have a good resolution for you, I still have to find one myself.