There are no legitimate health concerns associated with eating the tuna available today -- in sushi or other forms. Enjoy it without worry.
The front-page story by Marian Burros in the New York Times on Wednesday, January 23rd lacked critical perspective. She confused a legal/regulatory issue with a health issue.
The government sets standards on "acceptable" levels of mercury in fish. In calculating these levels, the Environmental Protection Agency (EPA) is very conservative. The Agency set levels that are some 10-100 times lower than the level that could potentially cause harm. Even the Food and Drug Administration (FDA) does not currently express any concern about the safety of tuna. It is those highly-conservative EPA standards that Burros quotes when she claims that the fish is "contaminated."
The levels now being found in tuna are well below levels that have ever been suspected of causing harm. Thus, while there may be a legal/regulatory issue here, with the fish violating the government's conservative tolerance level for mercury, that does not mean that a health hazard exists.
See also:
A New York Sun piece on the controversy, quoting ACSH's Jeff Stier.
A HealthFactsAndFears piece on the advantages of eating more fish despite such warnings.