Seihyouken on 19 January 2010
It seems many are vastly overexagerating Porsche's significance. They'll absolutely be missed and will almost definitely be the most prolific manufacturer to be absent from the game, however between the 90 manufacturers that are already confirmed for the game including Ferrari, Lamborghini, Bugatti, and McLaren as well as the ones that haven't even been announced yet, the lack of Porsches in the game shouldn't even really be an issue. This is only more true with you consider that Ruf will bring many of Porsche's designs with them into GT5 anyway.
