The purpose of this study is to investigate the effectiveness and safety of 12 weeks of treatment with R256918 in patients with Type 2 Diabetes Mellitus who are taking metformin. The primary measure of effectiveness is the change in concentration of glycated hemoglobin (HbA1c) during treatment. Glycated hemoglobin is a substance in red blood cells that is formed when blood sugar (glucose) attaches to hemoglobin and is a measure of diabetic status. Additional measures include fasting glucose, and lipid levels, and body weight. Safety assessments performed during the trial include laboratory tests, vital sign measurements, and adverse event reporting.