As long as the bag does not get immersed in water, Billinghams (completely closed!) are water-proof; LowePro's Magnum and smaller bags from the same series have been water-proof in the past, guess they still are.
I am just building my own from a Timbuk2 Messenger bag (the Pro Series XXL to be precise), which is water-proof as long as it is closed. I am still looking for a way to get a sensible interior, which can hold my gear in fixed positions safely. Obviously smaller Timbuk2s from the Cordura or (better looking to me) Ballistic Messenger series are another good choice for smaller gear.