Found this today posted on Amazon. I got it pricematched at my local Best Buy since I had $20 in reward certificates, so I got the game for $8. This version has the original game and ALL of the DLC released for it.
http://www.amazon.com/Forza-3-Ultimate-Platinum-Xbox-360/dp/B0046CC4R0/ref=dp_ob_title_vg
http://www.amazon.com/Forza-3-Ultimate-Platinum-Xbox-360/dp/B0046CC4R0/ref=dp_ob_title_vg