Secondary objectives: 1) Compare varenicline to placebo for smoking cessation efficacy after treatment at week 52; 2) Compare varenicline to placebo for effects on weight gain in participants successfully quitting smoking; 3) Compare diabetic outcomes in quitters vs relapsers at week 24 and week 52; 4) Examine predictors of abstinence at week 24 and at week 52; 5) Summarize safety data for 12 weeks of treatment with either varenicline or placebo.