Until the FDA and EPA awaken to the dangers, there's little that susceptible individuals can do to protect themselves except to stop ordering fresh tuna (raw or cooked -- canned yellowfin is generally safe). Sushi restaurants should post warnings. (California Proposition 65 requires them to do so, but it's not being enforced.) New technology allows for rapid mercury screening of fish, and conscientious seafood restaurants of all types ought to demand that their suppliers test tuna and other affected species before selling it. Beyond that, look at the chart above and decide for yourself. Basically, don't feed your kids tuna rolls or eat them if you're pregnant or might get pregnant -- there are plenty of other fish in the sushi case.
(This report derives from data provided by Eli Saddler, public health analyst for GotMercury.org.)