Study Comparing 0.5mg of Ranibizumab and Higher Doses in the Treatment of Clinically Significant Diabetic Macular Edema

0.5mg or 1.0mg intravitreal injection given monthly for 3 months then every other month if needed, for the first year.Patients may be treated monthly as needed. Patients in Cohort 1(the first 30 patients)will be eligible to receive treatment as needed with 2.0mg ranibizumab starting at month 24. Patients in Cohort 2(the remaining 20)will be eligible to receive treatment as needed with 2.0mg ranibizumab starting at month 12.Patients may receive 2.0mg ranibizumab for a period limited to 24 months following administration of the first 2.0mg dose. Patients completing 36 months will be eligible to be followed for an additional 24 months. Starting at Month 36, patients will be able to receive monthly, the assigned dose that they received/or could have received(if re-treatment criteria was met)at Month 35. 2/1/12 - the 2.0mg dose has been discontinued, all subjects will transition to the 0.5mg dose.