Also found this:
Mondale's campaign was already far behind the Republican ticket when Ferraro joined the ticket, and one issue that hurt her credibility was her disclosure of her husband's tax returns. In July 1984, she said she would release both her and her husband's tax returns. Yet a month later she backtracked and said she would release only her returns. Then she backtracked again, saying her husband would release "a financial — a tax statement" on August 20. But she must not have consulted her husband, because Zaccaro initially refused.
Reminds me of a current controversy. Let's find out what the Clintons have in their returns before we get a similar problem.
UPDATE: Media Matters makes the excellent point that McCain hasn't released his returns either, while receiving much less scrutiny. Obama would be well-advised to go after both of them at the same time over this issue.