I have seen Honda do this in the past. They have a magic computer that guess-timates how much mileage you have on the car and kicks out a letter reminding you that a service is coming up soon. That being said however, Hondas are due in every 7,500 miles for an oil change so it's pretty easy to extrapolate how fast you put miles on your car by looking at the service history.

As far as I know, the Crossfire does not have a scheduled 20,000 mile service, only service A and service B. Depending on the mileage, there will be certain extra tasks beside the basic oil change & tire rotation. I think your dealer is trying to "up sell" you on services.