+1 for Daytona (Road course). The Rolex 24 Hours of Daytona is usually a cold event with rain at some point during the race every year. It would be great to see that recreated in GT5. It would also be cool to see all the cool stuff around Daytona like the RV's and campfires and the big Ferris Wheel by "The Kink" turn in the infield section of the track.
The NASCAR boys don't race when it rains. Their July 400 miler is a night race though. I used to live in Central Florida and it'll pour down rain every day at about 16:00 but it rarely rains at night. Since they made that race a night race, it hasn't been affected by weather as much. Practices and qualifying for that race are almost always affected by weather, as those are still held during the day.