From the Sun-Times Oprah blog:

Oprah pal Gayle King told Entertainment Tonight that Winfrey will be there during Obama’s acceptance, however, there are no plans for her to take the stage at the rally. “Oprah has not been asked to introduce him, is not expecting to be asked to introduce him and I would doubt that she would want to introduce him. She thinks this is HIS moment. I know that she is planning to be there,” she told ET.