A couple of people have mentioned Carradice bags. I know you're carrying a laptop, which a Carradice bag probably isn't that great of a size/shape for (maybe one of the larger ones) but you might think about using a Carradice anyway, and carrying your computer on your back. There are plenty of elegant, compact backpacks available for this that won't weigh you down, and you can use the Carradice for everything else. If you're going to use a Carradice for commuting, I highly recommend the SQR mount, which makes it a piece of cake to take it off and carry around. Wallingford, Harris, Peter White, etc, should all have them. The advantage of carrying your computer on your back is that it's subjected to a lot less shock from bumps than if it were attached to the bike. If you want to carry it on the bike, Arkel makes a really nice, secure pannier for a laptop. My better half has one, and it's one of those rare items that I don't feel the need redesign in my head when I look at it. But for simplicity and good looks, and not needing to attach a rack, you can't beat a Carradice. The straps are kind of a pain to undo all the time, but the SQR mount takes care of that.
Emily O'Brien commuting daily from Medford, Massachusetts, USA