The MLB website says:
The Honorary Bat Girl experience will vary by MLB Club, the winner experience is not guaranteed, but will include the following:
One Honorary Bat Girl or Boy per team will be recognized on the field on Mother’s Day 2012
(An alternative date will be chosen for teams who are not home on Mother’s Day 2012)
A ticket to the game for themselves and a guest
Special MLB pink ribbon merchandise, including a pink bat that will be mailed at a later date