This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
Whether you have an expensive membrane or an inexpensive coating lurking
behind the face fabric of your jacket, the moment the fabric "wets out",
you're in danger of anything from drastically reducing performance to
turning your jacket into something with the breathability of a bin liner.
It's easy to see when this happens. The water no longer beads up and rolls
off the surface of the fabric, and you'll see it soaking into the material
in patches. The fabric is still waterproof (apart from pressure points -
see above), but its breathability will be greatly impaired. The answer is
to keep your jacket clean, following any washing instructions to the
letter, and maintain the water repellent finish on the outside.