This 2 arm study will assess the efficacy, safety and tolerability of taspoglutide compared to placebo in obese patients with type 2 diabetes mellitus inadequately controlled with metformin monotherapy. Patients will be randomized to receive taspoglutide (10mg sc once weekly for 4 weeks followed by 20mg once weekly) or placebo sc, in addition to their prescribed, pre-existing metformin therapy.After the first 24 weeks, patients on placebo will be switched to taspoglutide 20mg once weekly (after 4 weeks on taspoglutide 10mg once weekly) The anticipated time on study treatment is 12 months, and the target sample size is 100-500 individuals.