Though that's obviously not a confirmed date, it is a plausible one. "March 29" is a little too random to be a placeholder date, and is also a Sunday, a popular release day for big-name titles. Even if the franchise has reached the peak of its popularity, we still feel pretty confident about calling Guitar Hero: Metallica "big-name."
Meanwhile, Activision had no qualms about revealing release details for Europe, informing Eurogamer that the game would reach the region in May.
Source: Possible North American date [Via Destructoid]
Source: Confirmed European date