EA doesn't own the Porsche license. Microsoft does. EA used to have exclusive licneses to Ferrari, Lamborghini and Porsche, but when all their contracts were about to run out a few years ago, none of them chose to renew it because they were missing out on other opportunities like GT and Forza. However, Microsoft bought the exclusive license to Porsche. Ferrari and Lambo aren't doing exclusive licenses any more. (Although, in Lambo's case it's from a company that said that the Bugatti Veyron would NEVER be in a video game EVER) (huh...kinda like NASCAR. Seems like ever exclusive license that EA touches, the company ends up not doing them any more.)
But, because it's Microsoft that owns the license. I seriously doubt GT will ever get the rights to put Porsche in a GT title. The only way they can is to pay Microsoft. Which is a possibility since there's a Bandai Lexus in GT5.