update! what that other guy said was accurate, i got this yesterday:
Dear Valued Customer,
Thank you for pre-ordering Madden NFL 10 from GameStop / EB Games. It has come to our attention that a previously sent email contained incorrect instructions for downloading the Madden NFL 10 Demo. Please disregard those instructions and follow the correct instructions listed below.
The GameStop exclusive demo with four 5-minute quarters, featuring a Super Bowl rematch between the Pittsburgh Steelers and Arizona Cardinals, is scheduled to go live on Thursday, July 23 by 9pm EST.
Please do not try to download the demo before 9:00 pm on Thursday July 23. Doing so will result in an error message and could possibly deactivate your demo code. Anytime after 9:00 pm on Thursday July 23 you may follow the instructions below to download your Madden NFL 10 Demo.
1. Sign in to Xbox LIVE
account from the Xbox Dashboard, and then select the Marketplace blade.
2. Select “Redeem Code”
3. Use your controller and the on-screen keyboard to enter your key code, then select “Done”
4. Your playable demo will download to your Xbox 360
Hard Drive!
Your unique voucher code is
Thank you,
GameStop Customer Service