The purpose of the study is to test the hypothesis that oral administration of ruboxistaurin will reduce the occurrence of sustained moderate visual loss (SMVL) in patients with clinically significant macular edema. SMVL is defined as a 15 letter or more decrease from baseline in best-corrected Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity that is sustained for the patient's last 6 months of study participation. The SMVL data from this study will be combined with the SMVL data from Study B7A-MC-MBDL for the purpose of comparing ruboxistaurin to placebo.