I have the same dilemma, i made a couple of threads about what type of bag to buy (a poll) and what type of sleeve to buy. In the end i have setted, or buying a generic anti-shock case for my macbook, from ebay, i figure its just like a shinza knock off but will fit my macbook as well as a dakine Duel backpack (has a built in padded laptop sleeve). This combo allows me to take my books and laptop to school, protects well, was all under 100$ Canadian, and doesnt advertise what so ever that i have a laptop with me!
Its perfect mix of form and functionality, and a great price.
So just buy a sleeve and a backpack that has a laptop compartment. But NOT a dork bag built for a laptop.. they usually tend to be a giant black square with bulges everywhere...
No offence to anyone, i am a dork, i just try to hide in public 😉