I have a pair of Axiom Typhoon panniers. They say "waterproof" on the bag, are made primarily of a plasticky sort of material, and nothing has ever gotten wet inside, even during rides in torrential rain, which I've made somewhat regularly this year. These get the job done, and you can definitely fit a laptop, though I'd wrap mine in my work clothes just to be safe.

You can see 'em here: http://www.axiomgear.com/product/bag...uct.php?id=406