I gave the lack of Ferraris and Lamborghinis a thought. Many people suggested that more models could be unlocked=very possible but kinda unlikely because when you choose a manufacturer in GT5, it says sth. like: Cars you can purchase with your current level: number x of models, that, for me at least, means that all models are shown already, but not all can be bought, that however applies only to the premium cars.
Let me explain: Others suggested more models could be purchased through the used car dealer, which in my eyes is not only very possible but also very likely, here's why: I saw video footage of a Porsche 911 GT3, a Lamborghini Diablo (1990-road car), a Lamborghini Diablo Nomad JGTC Race Car (Japanese version) and a Bugatti EB110 in Gran Turismo 3. Now I am very sure that they modeled some cars of that makers for themselfes to play with them at PD, but did not implement them in the game because they simply lacked the licence. You remember that during the GT4 making of, PD test drove a Ferrari 360 and a 911 Porsche? I think that is another sign that they modeled these cars either for GT3 or GT4, but of course only for themselfes, as I already said. That is the reason we did not see a 360 as a premium I think.
Well, the conclusion is, they modeled lots of such cars for GT3 and 4 and now these cars are coming as standards to GT5, I'm almost convinced about that.