Remedy Interactives Matias Myllyrinne has revealed that the first DLC for Alan Wake will be free, but only to first time buyers.
Speaking via Twitter the developer said,
Our first Alan Wake Special (downloadable content) will be free for first time buyers.
We are unsure how they will figure out who the first time buyers are, unless they are using a system similar to EA with games such as Mass Effect 2 and Battlefield Bad Company 2.
Update: Microsoft have revealed (via Kotaku) that a token will be included in the game for anyone purchasing the game brand new, this will allow you to receive the content for free once it is released.