http://www.treasury.gov/press-center/press-releases/Pages/jl2153.aspx

“The U.S. Department of the Treasury and the Internal
Revenue Service (IRS) today ruled that same-sex couples, legally married in jurisdictions that recognize their marriages, will be treated as married for federal tax purposes. The ruling applies regardless of whether the couple lives in a jurisdiction that recognizes same-sex marriage or a jurisdiction that does not recognize same-sex marriage.”