"All Legal Same-Sex Marriages Will Be Recognized for Federal Tax Purposes."

The U.S. Department of the Treasury adopts its position in response to the Supreme Court's DOMA case. This in my view is unquestionably the correct response. It means that if a couple marries in a place that permits same-sex marriage, they'll be considered legally married regardless of where they currently reside and whether that place recognizes same-sex marriage.

“Today’s ruling provides certainty and clear, coherent tax filing guidance for all legally married same-sex couples nationwide. It provides access to benefits, responsibilities and protections under federal tax law that all Americans deserve,” said Secretary Jacob J. Lew. “This ruling also assures legally married same-sex couples that they can move freely throughout the country knowing that their federal filing status will not change.”

As for past tax years that are still open under the statute of limitations, you're given an option to file an amended return and be accepted as married, but you don't have to. So, if your tax bill would be less being considered unmarried, even when you were, you get to keep that advantage you had for those years. If you'll pay less by filing as married, you should file the amended return. Going forward, if you're married, you're married for federal tax purposes. You can't move to a state that doesn't recognize your marriage as a way to reduce your tax bill.