The developers of Excite Truck are currently working on a project for Nintendo that is so secret that computer screens must be darkened when deliverymen stop by.
Locally Grown Northfield, a blog that focuses on the town of Northfield, Minnesota, had an interview with Monster Games' founder Richard Garcia last month. In the interview, Garcia confirmed that the small development house has been "working on a top-secret video game for Nintendo" for the past two years. The unannounced game is reportedly almost finished and is so secret that "the designers must darken their monitors when the bottled-water deliveryman enters the office."
Monster Games developed the Wii launch game, Excite Truck, and has a history of developing racing games. The obvious guess at what this secret project is would be Excite Truck 2, but it could be another Nintendo racing IP, like F-Zero, or a new IP all together. Since the game is almost finished, details should be available soon enough.