You would have to eat quite a bit of tuna to worry about it. The kind that has the most mercury is canned white albacore. Pregnant women need to worry about it. Keep eating tuna, it is tasty and very good for you!
*edit* I found an interesting table that actually gives you the recommended amount of tuna you should eat based on your weight. Looks like I can eat 1 can every 3 days without worry (which is about how much I eat anyhow). Anyway, I still think its all a crock of shit.
http://www.nrdc.org/media/pressrelea...1211.asp#table