The closest thing to a true waterproof case that's still functional day-to-day is a two-stage solution.
First, a water-resistant sleeve (sfbags is a good suggestion, and there are others).
Second, a water-resistant bag (I use a Timbuk2 messenger).
My sleeve + messenger combo has weathered many rainstorms while I cycle to and from work every day. No problems at all. If you decide to go with the Timbuk2, I'd opt for the regular messenger over the laptop bags they sell.