Borderlands 2 will be out in North America on September 18, and internationally on September 21. That date might be enough incentive for you to pre-order -- it's so far away, and you might forget! -- but 2K offered initial incentive, in the form of the "Premiere Club."
Premiere Club members will get a unique "Gearbox Gun Pack," a "Golden Key" item to unlock something in the Golden Sanctuary Loot Chest, and the "Vault Hunter's Relic." These are all in-game items, by the way. 2K didn't really build a "Golden Sanctuary" and send out keys to it.