1800PocketPC has just noticed that Zombies!!! may be one of the first Windows Phone 7 games with downloadable content.
Xbox360Achievements.org lists the game as offering an expansion pack which will add 50 more gamer points and 5 new achievements, listed above.
1800Pocketpc expects the DLC to arrive in the next few months. Hopefully this means Microsoft is close to solving the in-app purchasing issue for developers, a major source of revenue on other platforms.