Separate from this promotion, the Green Day Rock Band product page already has a number of bonuses listed: a $10 off coupon code delivered when the game ships, a code to export the tracks to RB/RB2, and guaranteed street date delivery if you order overnight shipping.
This promotion says that its $10 off coupon code ships on 5/26/10, which is before the game ships. So hopefully, this means you'd get 2 separate $10 coupons for this.
