For AppHarbor use, this will use the config file by default but fall back to the corresponding environment variable if you use the value {ENV}

Config.cs

    public interface IConfig
    {
        string Get(string key);
    }

    public class Config : IConfig
    {
        public string Get(string key)
        {
            var fromConfig = ConfigurationManager.AppSettings[key];
            if (String.Equals(fromConfig, "{ENV}", StringComparison.InvariantCultureIgnoreCase))
            {
                return Environment.GetEnvironmentVariable(key);
            }
            else
            {
                return fromConfig;
            }
        }
    }

TheMovieDbService.cs

public class TheMovieDbService
{
  private readonly string _apiKey;

  public TheMovieDbService(IConfig config)
  {
    _apiKey = config.Get("TheMovieDB_API_KEY");
  }
}

web.config

<!-- usage -->
<configuration>
  <appSettings>
    <add key="TheMovieDB_API_Key" value="{ENV}"/>
  </appSettings>
</configuration>