If you go in the game marketplace, then game videos click on the first Red Dead Redemption.
If you read the description it says
"Xbox live Gold or Silver members who download the multiplayer free roam trailer on Xbox Live Marketplace between April 26th 2010 and may 10th will receive a 400 Microsoft points token and a theme token.
For complete rules go to http://www.xbox.com/en-GB/RDRgiveaway
Not sure if it works as obviously not those dates yet, would hope its true as its not jsut written online but actually on the xbox live marketplace.