And, indeed, there are very few choices out there. Vaude made several waterproof packs in the late 1990s, but has mostly abandoned that effort except for a messenger-style bag called the H2O Proof ($130 U.S.; www.vaude.de).
Why not get waterproof panniers? Ortlieb offers some excellent choices in this category. The Back-Roller Plus ($180 U.S.; www.ortlieb.de) can be used as a backpack with a strap system that makes for a superb bike-commuter piece. And it's totally waterproofI was touring with Ortlieb gear a month ago, and it withstood some real downpours. And I should think the weight of a laptop would be better placed low in a pannier, rather than on your back.
Otherwise, you could always buy a waterproof case for the laptop and stick the whole thing in a regular pack. Pelican makes a waterproof hard case for $105 U.S. (www.pelican.com). Or, get a kayaker's dry bag and put the laptop in that, then the dry bag into your pack. SealLine makes great, totally waterproof bags that shouldn't set you back more than about $25 U.S. (www.cascadedesigns.com/sealline).