CBS is reporting Zimmerman has been arrested following a disturbance at his home: http://www.cbsnews.com/8301-504083_162-57612824-504083/george-zimmerman-arrested-following-disturbance-at-fla-home/
UPDATE: CNN is reporting that the arrest is for pointing a shotgun at his girlfriend. Aggravated Assault with a firearm carries a 3 year mandatory minimum sentence in Florida. The cases are notoriously hard to prove, as they are often he said/she said scenarios with little physical evidence. There is also a criminal mischief charge (destruction of property), but there are not any facts available yet.