Digital Photography School has some good advice for how to choose a camera bag. They also have a great round-up of their reader's favorite camera bags (with links to Amazon where you can read reviews of each one.).
As I mentioned before, I currently use a Crumpler 4 Million Dollar Home for my Nikon D40 and am very pleased with it. This bag is far more comfortable than any other camera bag I've owned. It's not always ideal for travel, however, as it is too large to easily fit into other luggage and too small to carry anything but a camera and a couple accessories.
I increasingly like the idea of hacking another bag into a camera bag. I tried doing this with my Mountainsmith Day Pack when I went into DC to see the Cherry Blossoms with mom and dad. It worked like a charm.