I've had tracks go from wet to dry, but I didn't really see a line.
I was, however, racing in the wet at Monza yesterday. It stayed wet all race, but the racing line was visible. When braking on the line, or off the line, the car stayed in a straight line, but braking with one side on the line and the other side off the line caused the car to pull to one side. I thought that was a nice effect.