Hmmm
I think that probably two or three 6 oz. cans a day is the most you'd want to eat. While it's true that tuna steaks contain higher concentrations of mercury (because tuna steaks come from larger fish that have had more time to accumulate toxins) canned tuna still has a significant amount.
Your best bet is to eat Chunk light, also, as it's usually made from very small fish (solid albacore is made from larger).
Either way, I don't think there have been many studies that prove tuna consumption to be THAT harmful.