Title says it all! Here is the link –
Xbox LIVE Marketplace
Sign into your Xbox Live account, add it to your download queue and voila, it'll download it when you sign into Xbox Live from your Xbox 360. I don't see any expiration on this offer but get it while you can.
Update: This does not seem to be working for everyone any more so it is quite possible it was a goof-up by Microsoft. Regardless, plenty of reports that people have been able to download, successfully play the game and verified it is the full version.
Source: Xbox LIVE Marketplace
