One other option I forgot to mention is something I do with my hiking pack:

I can't afford a waterproof pack and it's always been a royal pain for me to pull out the integral rain cover because you can't do it without taking your pack off (unless you have some friends with you).

I load everything inside the pack which is succeptable to water inside silnylon roll top waterproof bags and put THOSE in the pack. They're light, relatively inexpensive, and keep everything but the pack itself dry.

Mountain Equipment Coop has just started selling some large enough to (and designed to) fill the entire inside of a pack for this purpose. I like several smaller color coded ones personally it makes organization easier.