Due to the fact that the #5 Force One Viper sustained major damage at Orscherslaben last time out the team has decided that they cannot repair the car in time for Estoril or Monza.
http://www.fiagt.com/index.php?event=9&season=2003 Hopefully the team will be back though with the Vipers next year, or if not the Viper some other car, the number #4 car will be at Estoril as normal though.
Just speculaton here but what are the chances that Force One never get around to repairing the car, simply because they plan on running another machine next year, ie a Ferrari 575, Lambourgini, Saleen S7R, or even a Lister? I forget where but I think it was AUTOSPORT magazine that they said their may be 6 Lister Storms racing next year. But as I said this is all just speculation.