Unless you already own the Bullfrog classic, there's no reason to miss out on this deal. It seems many people are taking the site up on the offer -- requesting your copy currently gives the message "You'll get your game soon! Minion, due to high demand, we will tell you when your free copy of Dungeon Keeper is ready via email in a little bit."
GOG.com, you can be my valentine any time. Also, Dungeon Keeper 2 is on sale for $1.49.