Just announced free DLC at launch - two tracks and 10 cars.
1957 Ferrari 250 Testa Rossa
1960 Chevrolet Corvette
1964 Aston Martin DB5 Vantage
1965 Alfa Romeo Giulia Sprint GTA
1965 Shelby Cobra Daytona Coupe
1967 Lamborghini Miura P400
1969 Dodge Charger
1971 Nissan Skyline 2000 GT-R
1980 Fiat 131 Abarth
1981 BMW M1
Tracks are Benchmark High Speed Ring (an airfield) and the Sidewinder Proving Grounds (which I believe is the 18-config test track in Forza 2).
EDIT: Beaten to it, but my post is way more awesome