Both Polyphony Digital (Gran Turismo developers) & Turn 10 (Forza developers) have made tribute videos to the new Ferrari 458 using their latest game engines for PS3 (GT5) & Xbox 360 (Forza 3).
So now those who said "Haha GT5 is delayed" can see where the extra time being taken has been spent.
The Forza 3 Video
The Gran Turismo 5 Video
PS: The car isn't in Forza 3, it is part of some downloadable content which you will pay for. It is part of Gran Turismo 5 out of the box.