In the UK if you pre-ordered from PLAY.COM you were given a redeem code for xbox LIVE.
I just went on there and downloaded the file.
As far as I know it's the same pack you guys will be getting in the US.
It gives you the Gallardo Coupe, R34 Skyline and an Audi.
Just in case you don't believe me:
Play.com told me the code would probably not work till the 13th but it worked when I tried it.