Seems like early May.
Here are the latest two updates from SportingNews:
April 25, 2004 - 9:46 p.m.
Nixon, who hasn't played this season because of an injured back, is to face a pitcher Monday for the first time since getting hurt.
Fantasy Source spin: At least this is a step in the right direction for him.
April 24, 2004 - 3:38 p.m.
Nixon took batting practice against minor-league pitchers, the Boston Globe reported. Nixon could be close to going on a minor-league rehab stint.
Fantasy Source spin: An early May return continues to be the timeline for a return.