Well, I might as well be the first... have you considered getting the bag off of your back and attaching it to a rack? Assuming this is even possible on your bike there are some great panniers available that would protect your gear from the elements while keeping your back cooler by riding on the rack. I have an Arkel pannier and I can't say enough good things about their products. Their Urban series also includes a messenger bag: http://www.arkel-od.com/us/all-categ...briefcase.html. It's laptop compatible, comes in black, has 1100 cubic inches of volume, and there is an optional bright yellow rain cover available. The bag is about $140 USD and the rain cover is another $20. I have their "Commuter" bag with rain cover; I've had it for about 3 years and it has seen many commuting miles through all kinds of weather (including winter) and it still looks and works like new.

As for pants I use some cheap PVC rain pants that I got from an outdoors store. They are great for shorter commutes. For long rides I would opt for something more cycling-specific.