This study is a placebo-controlled study in patients with Type 2 Diabetes Mellitus who are either taking no diabetes medication or who are taking metformin only. This study will investigate the safety, tolerability, and efficacy of Albiglutide (GSK716155) and will measure the levels of Albiglutide (GSK716155) in the bloodstream when it is given for 16 weeks. As a comparison, some subjects will receive exenatide instead of Albiglutide (GSK716155). The study will involve weekly visits for 17 weeks,and less frequent follow-up visits for an additional 10 weeks. Assessments include repeat blood sampling and monitoring of any side effects.