Lightweight but durable/protective laptop backpack?

So, I recently bought a ZeroShock sleeve for my Vaio SR, but would like a bit more protection with an actual laptop compartment in my backpack. I usually do carry a large amount of papers/notebooks & textbooks, so I'm looking for something that won't add significantly to the weight already.

Which laptop backpacks have good protection for the laptop (on the bottom, on the sides and on the top - basically all around the laptop) and are comfortable to wear while not being a giant heavyweight? I considered the Swiss Gear backpacks, but they seem pretty heavy and also quite thick. I would prefer a backpack I can fit under the airplane seats, with bonus points for being expandable (i.e. if I don't put a lot of stuff in it, I don't want it to still be hugely thick).

Does such a backpack exist?

 

You may want to invest in a laptop sleeve. They can be bought in most popular laptop sizes. They have lots of padding around all sides, to keep the system protected from whatever else you put in your backpack.

K-TRON

 

Yeah, I do have a laptop sleeve, but I'm also looking into a bag with a dedicated laptop compartment if only to keep it from sliding around all over the place or bumping up against other stuff I keep in my bag. A bit more protection would give me some peace of mind, but I'm willing to let that requirement slip a bit if I can find a nice, lightweight bag with a dedicated compartment that's not too thick.