I really like my Waterfield bag (http://www.sfbags.com). I have the medium Cargo model and use it as my everyday briefcase, and as a laptop bag when traveling. They also make sleeves that are custom designed to fit specific Apple laptop models (including the MacBook).

Their bags aren't cheap, but they're very well made, and they have superb customer service. They even customized my Cargo with D-rings and a removeable shoulder strap for an extra $15--and still offered me the money-back satisfaction guarantee.

Plus, what FlyerTalker can possibly resist a bag that has an airline seat belt clip on the front of it?