Why is Sarah Palin the only candidate who doesn't have to release her tax returns?

Barack and Michelle Obama did. John McCain did (though Cindy McCain gets to censor her own and only release what she wants to, since they file separately and the IOKIYAR rule applies). But Sarah Palin, who thinks she's running for President, has not released hers:

As election day comes ever nearer, Democrats and open-government advocates are pressing for the GOP vice presidential candidate to release her tax filings, a campaign tradition that extends at least to the post-Watergate era.

Instead, Palin has to date declined to share the documents, becoming part of an ever-more-select historical group of candidates who waited until this late in a campaign year to release their tax filings.

[snip]

A spokesman for Palin Thursday reiterated the campaign's stance on the matter. "We plan to release Governor Palin's tax returns well before the election," said Taylor Griffin.

There's just over a month until the election. Does it depend on what the meaning of "well" is?