The two things that kill you when you are packing are weight and bulk. You can avoid both, and still have a waterproof jacket, if you get one that reduces down to nothing and can be stuffed in a little case.
I picked up one from Sierra Designs a few weeks ago at REI when I needed to spend my yearly rebate on something. (REI's not exactly cheap, but at least you get some of that money back in you're a member.) The thing was hanging on a little hook, in a nylon case a little smaller than a 12-ounce soda can. The whole jacket fits into there once it's rolled up, so it takes up very little space in your pack or suitcase.
I took it for a test run last weekend when I went on a short trip. It was easy to fit into my pack and I got to try it out when a rainstorm hit. The jacket repelled the water surprising well for something that weighs next to nothing. It was $30, about the price of a low-end fleece, but far easier to pack. Definitely pulls its weight. Search "Sierra Designs Microlight Jacket" at REI Men's Shop
Or get the women's version here: Sierra Designs Microlight Jacket - Women's