I have a Weber Genesis Silver C. Love the grill, even have the rotisserie basket/motor for it. I've replaced the grates with the porcelain-coated cast iron.
I got the plans for a smoker here:
http://how-to-plans.com/bbq_smoker_plans.html
Hopefully after I'm done redoing the lawn and putting in a fence this summer I'll build it as a project with my father-in-law.