XiaoBaiTu, 其他大俠 Question about changing F1 to H1, please help!

My wife has had H1-B before for 2 years, then she went back to school, so her current status is F1. She is using her OPT to work now, her employer is planning to file her H1-B soon. My question is if her OPT card expires on May 1, 2008, can she work for her employer if she receives her H1-b receipt notice after OPT expires? Or she has to receive the H1-b application approval notice before OPT expires in order to continually work? BTW, I know that she doesn't have quota problem since she has applied H1-b before. Please help me, thanks.