The "First Access Club," i.e. those who preordered the game from 2K and Gearbox (or bough the Borderlands GOTY edition) will get "the very first Duke Nukem Forever multiplayer downloadable add-on for free!" So, the lack of specifics means maybe it isn't a map, but probably a skin or a weapon of some kind. Duke Nukem Forever kicks ass and chews bubblegum June 14.
Update: A source familiar with the situation says it is "a full DLC. Not some silly skin."