Brenthaven
I have two Brenthaven bags for my 12" PB. One is the custom backpack that the Apple Store sells, the other is the Deluxe Slim shoulder bag that is basically the same as the one the Apple Store sells, except that the laptop sleeve is an adjustable sleeve, and not custom fit for the PB. These are not the most fashionable or svelte bags, but they will really protect a computer (the earlier post saying that there is about 4" of padding on the backpack for the laptop was not kidding) and are very, very well made. The nice thing is that they show up on eBay with some frequency. I bought the backpack for $70 and the shoulder bag for $40, both new, off eBay. If you are patient and want to save some money, that's the way to go.
That said, I used to just throw my IBM Thinkpad into a flimsy Tumi sleeve and stuff it into my Tumi briefcase (not a computer case) -- or if I had a lot of papers, a document case -- and had no problems whatsoever after dozens and dozens of business trips. Maybe I got lucky, many times, but that certainly worked. I was into traveling really light and couldn't have been bothered with a case.
Although they rarely get mentioned when the topic of bags come up up, the Tumi computer cases (which look like their briefcases, but are larger) are supposed to be really nice. They have a sling suspension system like the Tom Bihn bags. The downside is that they are very expensive, and I'm not sure that they will fit the 15" PB because of the width. Expect to pay $300 or more for a nylon bag, and lots more for leather. The upside is that they are, in my view, hands down the nicest, more versitile briefcases out there. Again, you can use the non-computer case versions with a sleeve, but it won't offer as much protection.