Coming early October, and looking absolutely fantastic. Got my preorder down for the Limited Edition already. Top Gear are heavily involved in the game, providing some commentary on some of the cars, their Test Track is also featured (along with the Kia Cee'd so you can compare yourself properly to the 'celebrities').
Looks incredible, I'll upload some screenshots later.
The game also is going to feature bonuses for people who have already played Forza 3, looks like it'll earn you bonus cars to get started with.
And if you have a preorder down, email proof of that and they'll send you some bonus stuff too when the game drops.