Madden NFL 11 on Xbox 360 has 50 standard achievements (1000 Gamerscore), all visible on Xbox.com and in-game. There are no hidden/secret achievements.
Check Xbox.com Official List or TrueAchievements for descriptions and unlock rates.
If you recall a "secret" achievement, you may be confusing it with another title (e.g., Madden 10 had "The Rookie" hidden until unlocked, but still listed).