This study is to demonstrate the safety and efficacy profile, in two different dose regimens of DU-176b, (an investigational new drug being tested for the prevention of stroke/systemic embolic events (SEE)), in individuals with atrial fibrillation. Patients will be randomized to one of three treatment groups: High Dose Regimen, Low Dose Regimen, & Warfarin. The expected duration of the study is 24 months.