There are a couple of things that you could use to try and remove the stain. Meguiars or Mothers make a clay kit that is very good at removing surface contaminants on a clear coat which is where the stain would be. Past claying that side, you could use a mixture of distilled white vinegar and water. Several detailers suggest such a mixture for water spots. Apply it to a clean car, rinse, and follow up w/ some wax. Good luck w/ that, you should be able to remove the stain w/o having to get anything re-painted.

Devandra