This 4 arm study will assess the efficacy, safety and tolerability of taspoglutide compared to sitagliptin and placebo in patients with type 2 diabetes mellitus inadequately controlled with metformin. Patients will be randomized to receive taspoglutide (10mg once weekly or 10mg once weekly for 4 weeks followed by 20mg once weekly), sitagliptin 100mg once daily or placebo, in a ratio of 2:2:2:1, in addition to their continued prestudy metformin treatment. After 24 weeks of treatment, patients on active treatment will continue on the same treatment and patients on placebo will be switched to taspoglutide 10mg once weekly or taspoglutide 20mg once weekly (after 4 weeks of taspoglutide 10mg once weekly). The anticipated time on study treatment is 2+ years, and the target sample size is 500+ individuals.