The most recent GenCon email newsletter detailed some of the goodies that Wizards will have available on the con floor.
First up, people who sign up for D&D Insider at the Wizards booth will receive a free D&D backpack. For those of you who really want to let your geek flag fly, I suppose!
They'll be pre-selling Adventurer's Vault 2 and the Dragon Magazine 2009 Annual, 100 copies per day, limit one per person.
Wizards is also heavily promoting their new free version of D&D Online, their Eberron-themed MMO. The game has changed to a free-to-play model, where you instead purchase special items and unlocks with real money, similar to many Japanese MMOs now. The GenCon promotion is that, when you buy any D&D product at the Wizards' booth, you get a free $5 DDO store gift card.
There's certainly going to be some nice incentives for the D&D fan at the con!