I've done a few wet weather races, actual enduros. 2hr N'ring at night with damage setting at heavy, fun times.
What's cool with the rain in GT5 is that there are varying levels of rain, it looks different, the real heavy rain will have notably bigger splash circles. When that happens your grip level drops further, so you gotta keep an eye out for it and be ready to drive extra careful.
To answer your original question, no there's nothing like a dry line that appears when the rain stops.
But there is still plenty of realism to how the wet track transitions to dry. You need to keep the wet tyres on for a while, you can't just change to drys right away, it'd be a bad idea. You can judge the wetness/dryness of the track by the amount of spray the cars kick up. If you're on the track alone, keep checking the rear view to see how much spray there is from your own car.
The weather feature of the game is very well done, it's a shame that most players don't like it and most organised series avoid it.